Wafer-scale techniques for fabrication of semiconductor chip assemblies

ABSTRACT

Semiconductor chip assemblies are fabricated by assembling flexible, sheetlike elements bearing terminals to a wafer, connecting the terminals of each sheetlike element to contacts on the chip, and subsequently severing the chips from the wafer to provide individual assemblies. Each assembly includes a sheetlike element and a chip, arranged so that the terminals on the flexible element.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 08/030,194, filed Apr. 28, 1993 as the national phase ofInternational Application PCT/US91/06920 filed Sep. 24, 1991. Said030,194 application in turn is a continuation of U.S. patent applicationSer. No. 07/765,928, filed Sep. 24, 1991, now U.S. Pat. No. 5,347,159.Said application Ser. No. 07/765,928 in turn was a continuation-in-partof U.S. patent application Ser. No. 07/673,020, filed Mar. 21, 1991, nowU.S. Pat. No. 5,148,265 and said application Ser. No. 07/765,928 was acontinuation-in-part of U.S. patent application Ser. Nos. 07/586,758,filed Sep. 24, 1990, now U.S. Pat. No. 5,148,266. Said U.S. patentapplication Ser. No. 08/030,194 is also a continuation-in-part of saidapplications Ser. No. 07/586,758 and Ser. No. 07/673,020.

TECHNICAL FIELD

The present invention relates to the art of electronic packaging, andmore specifically to assemblies incorporating semiconductor chips and tomethods and components useful in making such assemblies.

BACKGROUND OF THE INVENTION

Modern electronic devices utilize semiconductor chips, commonly referredto as "integrated circuits" which incorporate numerous electronicelements. These chips are mounted on substrates which physically supportthe chips and electrically interconnect each chip with other elements ofthe circuit. The substrate may be a part of a discrete chip package usedto hold a single chip and equipped with terminals for interconnection toexternal circuit elements. Such substrates may be secured to an externalcircuit board or chassis. Alternatively, in a so-called "hybrid circuit"one or more chips are mounted directly to a substrate forming a circuitpanel arranged to interconnect the chips and the other circuit elementsmounted to the substrate. In either case, the chip must be securely heldon the substrate and must be provided with reliable electricalinterconnection to the substrate. The interconnection between the chipitself and its supporting substrate is commonly referred to as "firstlevel" assembly or chip interconnection, as distinguished from theinterconnection between the substrate and the larger elements of thecircuit, commonly referred to as a "second level" interconnection.

The structures utilized to provide the first level connection betweenthe chip and the substrate must accommodate all of the requiredelectrical interconnections to the chip. The number of connections toexternal circuit elements, commonly referred to as "input-output" or"I/O" connections, is determined by the structure and function of thechip. Advanced chips capable of performing numerous functions mayrequire substantial numbers of I/O connections.

The size of the chip and substrate assembly is a major concern. The sizeof each such assembly influences the size of the overall electronicdevice. More compact assemblies, with smaller distances between chipsprovide smaller signal transmission delays and hence permit fasteroperation of the devise.

First level interconnection structures connecting a chip to a substrateordinarily are subject to substantial strain caused by thermal cyclingas temperatures within the device change during operation. Theelectrical power dissipated within the chip tends to heat the chip andsubstrate, so that the temperatures of the chip and substrate rise eachtime the device is turned on and fall each time the device is turnedoff. As the chip and the substrate ordinarily are formed from differentmaterials having different coefficients of thermal expansion, the chipand substrate ordinarily expand and contract by different amounts. Thiscauses the electrical contacts on the chip to move relative to theelectrical contact pads on the substrate as the temperature of the chipand substrate changes. This relative movement deforms the electricalinterconnections between the chip and substrate and places them undermechanical stress. These stresses are applied repeatedly with repeatedoperation of the device, and can cause breakage of the electricalinterconnections. Thermal cycling stresses may occur even where the chipand substrate are formed from like materials having similar coefficientsof thermal expansion, because the temperature of the chip may increasemore rapidly than the temperature of the substrate when power is firstapplied to the chip.

The cost of the chip and substrate assembly is also a major concern. Allthese concerns, taken together, present a formidable engineeringchallenge. Various attempts have been made heretofore to provide primaryinterconnection structures and methods to meet these concerns, but noneof these is truly satisfactory in every respect. At present, the mostwidely utilized primary interconnection methods are wire bonding, tapeautomated bonding or "TAB" and flip-chip bonding.

In wire bonding, the substrate has a top surface with a plurality ofelectrically conductive contact pads or lands disposed in a ring-likepattern, The chip is secured to the top surface of the substrate at thecenter of the ring-like pattern, so that the chip is surrounded by thecontact pads on the substrate. The chip is mounted in a face-updisposition, with the back surface of the chip confronting the topsurface of the substrate and with the front surface of the chip facingupwardly, away from the substrate, so that electrical contacts on thefront surface are exposed. Fine wires are connected between the contactson the front face of the chip and the contact pads on the top surface ofthe substrate. These wires extend outwardly from the chip to thesurrounding contact pads on the substrate. In the wire bondedassemblies, the area of the substrate occupied by the chip, the wiresand the contact pads of the substrate is substantially greater than thesurface area of the chip itself.

In tape automated bonding, a polymer tape is provided with thin layersof metallic material forming conductors on a first surface of the tape.These conductors are arranged generally in a ring-like pattern andextend generally radially, towards and away from the center of thering-like pattern. The chip is placed on the tape in a face downarrangement, with contacts on the front surface of the chip confrontingthe conductors on the first surface of the tape. The contacts on thechip are bonded to the conductors on the tape. Ordinarily, numerouspatterns of conductors are arranged along the length of the tape and onechip is bonded to each of these individual patterns, so that the chips,once bonded to the tape, can be advanced through successive workstations by advancing the tape. After each chip is bonded to themetallic conductors constituting one pattern, the chip and theimmediately adjacent portions of the pattern are encapsulated and theoutermost portions of the metallic conductors are secured to additionalleads and to the ultimate substrate. Tape automated bonding can providethe assembly with good resistance to thermal stresses, because the thinmetallic leads on the tape surface are quite flexible, and will bendreadily upon expansion of the chip without imposing significant stressesat the juncture between the lead and the contact on the chip. However,because the leads utilized in tape automated bonding extend outwardly ina radial, "fan out" pattern from the chip, the assembly is much largerthan the chip itself.

In flip-chip bonding, contacts on the front surface of the chip areprovided with bumps of solder. The substrate has contact pads arrangedin an array corresponding to the array of contacts on the chip. Thechip, with the solder bumps, is inverted so that its front surface facestoward the top surface of the substrate, with each contact and solderbump on the chip being positioned on the appropriate contact pad of thesubstrate. The assembly is then heated so as to liquify the solder andbond each contact on the chip to the confronting contact pad of thesubstrate. Because the flip-chip arrangement does not require leadsarranged in a fan-out pattern, it provides a compact assembly. The areaof the substrate occupied by the contact pads is approximately the samesize as the chip itself. Moreover, the flip-chip bonding approach is notlimited to contacts on the periphery of the chip. Rather, the contactson the chip may be arranged in a so-called "area array" coveringsubstantially the entire front face of the chip. Flip-chip bondingtherefore is well suited to use with chips having large numbers of I/Ocontacts. However, assemblies made by flip-chip bonding are quitesusceptible to thermal stresses. The solder interconnections arerelatively inflexible, and may be subjected to very high stress upondifferential expansion of the chip and substrate. These difficulties areparticularly pronounced with relatively large chips. Moreover, it isdifficult to test and operate or "burn-in" chips having an area array ofcontacts before attaching the chip to the substrate. Additionally,flip-chip bonding ordinarily requires that the contacts on the chip bearranged in an area array to provide adequate spacing for the solderbumps. Flip-chip bonding normally cannot be applied to chips originallydesigned for wire bonding or tape automated bonding, and having rows ofclosely spaced contacts on the periphery of the chip.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a semiconductor chipassembly. An assembly according to this aspect of the inventiontypically includes a semiconductor chip having a plurality of surfacesand having contacts on at least one of said surfaces. The assemblyfurther includes a sheetlike, preferably flexible, element havingterminals thereon, the terminals being electrically connected to thecontacts on the chip. Assemblies according to this aspect of theinvention are characterized in that the sheetlike element and at leastsome of said terminals overly one surface of said chip, said terminalsare movable with respect to said chip and in that resilient means forpermitting displacement of the terminals toward the chip, but resistingsuch displacement are provided. Most preferably, a compliant layer isdisposed between said terminals and said chip so that said compliantlayer will be compressed upon movement of said terminals toward saidchip.

The complaint layer may be incorporated in the sheetlike element, orformed separately therefrom. The contacts typically are disposed on thefront or top surface of the chip. The sheetlike element and terminalsmay overlie said front surface of the chip. Alternatively, the sheetlikeelement and said terminals may overlie the rear, or bottom surface ofsaid chip. The terminals on the sheetlike element can be connected tocontact pads on a substrate, as by solder bonding. Because theterminals, and hence the contact pads on the substrate overlie the chipfront or back surface, the assembly is compact. The ability of theterminals to move with respect to the chip in directions parallel to thechip surfaces provides compensation for differential thermal expansionof the chip and substrate.

The ability to accumulate movement of the terminals towards the face ofthe chip greatly facilitates temporary engagement of the terminals bytest equipment and hence facilitates testing and "burn-in" of theassembly before the same is mounted to a substrate. According to afurther aspect of the present invention the compliant layer includesmasses of compliant material interspersed with holes. Desirably, eachsuch mass is aligned with one of the terminals.

A further aspect of the invention provides method of making asemiconductor chip assembly including the step of assembling a flexible,sheetlike element having terminals thereon to a seminconductor chip andconnecting terminals on said sheetlike element to contacts on said chip.Methods according to this aspect of the invention desirably arecharacterized in that the assembling step is conducted so that saidterminals on said sheetlike element overlie a surface of the chip and inthat a compliant layer is disposed between said chip and said terminals.Most preferably, these methods are further characterized by the step oftesting the chip by establishing temporary electrical contact between aplurality of test probes and said terminals and utilizing said temporaryelectrical contact to actuate said chip. The compliant layer permitsdisplacement of at least some of said central terminals toward said chipduring the step of establishing temporary electrical contact. The stepof establishing temporary electrical contact preferably includes thestep of simultaneously establishing temporary contact between aplurality of terminals and a plurality of test probes rigidly connectedto a test fixture.

Further aspects of the invention provide components for assembly to asemiconductor chip including a flexible sheetlike element havingterminals thereon, characterized by a compliant layer underlying saidterminals. The compliant layer preferably includes masses of a lowmodulus material and holes interspersed with said masses of low modulusmaterial, said masses of said low modulus material being aligned withsaid terminals, said holes in said compliant layer being out ofalignment with said terminals.

A chip assembly according to a further aspect of the invention includesa semiconductor chip having a front surface with a plurality of contactsdisposed in a pattern on the front surface. The pattern of contacts onthe front surface encompasses an area, referred to herein as the"contact pattern area" on the front surface. The chip assembly accordingto this aspect of the invention also includes a sheetlike dielectricelement, referred to herein as "interposer", overlying the front surfaceof the chip. The interposer has a first surface facing toward the chipand a second surface facing away from the chip. An area of theinterposer overlies the contact pattern area of the chip. The interposerhas apertures extending through it, from the first surface to the secondsurface. The interposer also has a plurality of electrically conductiveterminals disposed in a pattern on the second surface of the interposer.At least some of these terminals, and preferably most or all of theseterminals, are disposed within the area of the interposer overlying thecontact pattern area on the chip. Each such terminal is associated withone contact on the chip.

The assembly also includes flexible, electrically conductive leads. Theleads preferably extend through the apertures in the interposer. Eachsuch lead has a contact end connected to the associated contact of thechip and a terminal end connected to the associated terminal on thesecond surface of the interposer. The leads and the interposer areconstructed and arranged so that the contact ends of the leads aremoveable relative to the terminals at least to the extent required tocompensate for differential thermal expansion of components. The leadsdesirably are flexible to permit such movement. Most preferably, theinterposer itself is flexible so as to facilitate such movement. Theassembly according to this aspect of the invention optionally mayinclude a compliant layer as discussed above.

The assembly incorporating the chip, interposer, terminals and leads maybe incorporated in a larger assembly including a substrate having a topsurface facing toward the second surface of the interposer.

Preferred chip assemblies according to this aspect of the presentinvention are compact and may be utilized with chips having largenumbers of input-output connections. The terminals on the interposer,and the corresponding contact pads on the substrate, desirably aredisposed in areas substantially the same size as the contact patternarea on the chip itself.

The flexible leads may be formed integrally with the terminals on theinterposer, or else may be separately formed fine wires. The leadsdesirably are curved to provide increased flexibility. The interposerdesirably is a thin, flexible sheet of a polymeric material such aspolymide, a fluoropolymer, a thermoplastic polymer or an elastomer. Inthis arrangement, flexing of the interposer facilitates movement of thecontact ends of the leads relative to the terminals and thus contributesto the ability of the assembly to withstand thermal cycling. Theassembly may also include a compliant dielectric encapsulant having alow elastic modulus, such as an elastomeric encapsulant, covering theflexible leads in whole or in part. The encapsulant may be provided inthe form of a layer, with holes in the encapsulant layer aligned withthe terminals on the second surface of the interposer. The bonds betweenthe terminals and the contact pads of the substrate extend through theseholes. The encapsulant protects the relatively delicate leads duringhandling and during service, but does not prevent flexing of the leadsor the absorption by the leads of relative motion of the chip andsubstrate during thermal expansion.

A chip assembly according to yet another aspect of the present inventionincorporates a chip having a front surface including a central regionand a peripheral region surrounding the central region, the chip havinga plurality of peripheral contacts disposed in the peripheral region ofthe front surface. The assembly preferably further includes a sheet-likedielectric interposer overlying the central region of the chip frontsurface. The interposer has a first surface facing downwardly toward thechip and a second surface facing upwardly, away from the chip. Theinterposer also has edges disposed inwardly of the peripheral contacts.For example, the interposer may overly only the central portion of thechip front surface. A plurality of central terminals are disposed on theinterposer and overly the central region of the chip front surface. Theassembly preferably also includes a plurality of peripheral contactleads connecting at least some of the peripheral contacts on the chipwith at least some of the central terminals on the interposer. Each suchperipheral contact lead thus has a central terminal end overlying theinterposer and connected to one of the central terminals and a contactend projecting outwardly beyond one of the edges of the interposer andconnected to one of the peripheral contacts. Each peripheral contactlead extends inwardly from one of the peripheral contacts to one of thecentral terminals on the interposer. The peripheral contact leads andpreferably the interposer as well are at least partially flexible sothat the central terminals are movable with respect to peripheralcontacts to accommodate movement caused by differential thermalexpansion. Here again, the assembly may optionally include a compliantlayer as discussed above. Desirably, the peripheral contact leadsinclude bent portions.

The peripheral contact leads and central terminals provide a "fan-in"arrangement in which the terminals on the interposer are disposed insidethe region bounded by the peripheral contacts on the chip. Typically,the peripheral contacts on the chip are disposed in one or two rowsalong each edge of the chip, in a generally rectangular pattern, so thatthe contacts on the chip are close to one another. By contrast, theterminals on the interposer may be substantially evenly disposed overthe second surface of the interposer. The central terminals may bedisposed in a so-called "area array". Accordingly, the distance betweenadjacent terminals may be substantially greater than the distancebetween adjacent contacts on the chip. The distances between adjacentterminals on the interposer may be large enough to accommodate solderbonding and similar processes which require substantial distancesbetween adjacent bonds.

Some or all of the peripheral contact leads may have outward extensionsprojecting outwardly beyond the peripheral contacts of the chip. Theassembly may include securement means for holding these outwardextensions. For example, one or more securement elements may be disposedoutwardly of the peripheral contacts, and each such securement elementmay be physically connected to a plurality of the outward extensions onthe peripheral contact leads. Each such securement element may be agenerally planar strip of dielectric material having an inboard edgeextending generally parallel to one of the edges of the interposer sothat each pair of parallel edges define an elongated slot between eachsuch securement element and the interposer, and each peripheral contactlead may extend across one of these slots. In this arrangement, theperipheral contacts of the chip may be disposed in alignment with theslots between the securement elements and the interposer. The securementelement may be physically connected to the interposer, as by bridgeelements extending between the securement elements and the interposer atspaced-apart locations around the periphery of the chip front surface.The securement elements, bridge elements and interposer may be formedintegrally with one another as a single, sheet-like unit. The securementelements provide physical reenforcement to the peripheral contact leadsduring the manufacturing operations and in service. Additionalterminals, referred to herein as "outside" terminals, may be disposed onthe securement elements, and may be connected to some of the peripheralcontacts on the chip by outside terminal leads extending across theslots, the inboard ends of the outside terminal leads being secured tothe interposer so that the slot and interposer cooperatively providereinforcement to the outside terminal leads as well.

These assemblies may be made by methods which include the step ofassembling a sheet-like dielectric interposer to the chip so that theinterposer overlies the central region of the chip front surface, theoutboard edges of the interposer being disposed inwardly of theperipheral contacts on the chip. When the dielectric interposer isdisposed on the chip, a first surface of the interposer faces downwardlytoward the chip and a second surface of the interposer faces upwardlyaway from the chip, and a plurality of central terminals on theinterposer overly the central region of the chip front surface. Themethod further includes the step of connecting a plurality of peripheralcontact leads between at least some of the peripheral contacts of thechip and at least some of the central terminals on the interposer, sothat each such peripheral contact lead extends inwardly from one of theperipheral contacts on the chip to one of the central terminals on theinterposer. The method may further include the step of assembling asubstrate having a plurality of contact pads to be assembled interposerand chip and connecting each of the central terminals on the interposerto one of the contact pads on the substrate.

The interposer may have prefabricated leads mounted thereon andconnected to the central terminals before the interposer is assembled tothe chip. In this case, the prefabricated contact leads are positionedon the chip when the interposer is assembled to the chip. Suchprefabricated contact leads may be electrically connected to thecontacts of the chip by thermocompression bonding or similar processes.Alternatively, the peripheral contact leads may be formed after theinterposer is applied to the chip, as in a wire-bonding step in which afine wire is dispensed and formed into a lead connecting the contact andterminal. Preferably, securement elements are provided as discussedabove with reference to the chip assembly, and the securement elementsare connected to the interposer before the interposer is placed on thechip. In this case, the securement elements may support theprefabricated leads during the step of placing the interposer on thechip.

A semiconductor chip assembly in accordance with yet another aspect ofthe invention includes a semiconductor chip having oppositely facingfront and rear surfaces with edges extending between these surfaces, thechip having contacts on the front surface. The assembly further includesa generally sheet-like element referred to herein as "backing element"underlying the chip, the backing element having a top surface facingtoward the chip and a bottom surface facing away from the chip. Acentral region of the backing element is aligned with the chip. Thebacking element is provided with terminals. At least some, andpreferably all of the terminals on the backing element are disposed inthe central region, so that the terminals underlie the bottom surface ofthe chip. The assembly in accordance with this aspect of the presentinvention further includes electrically conductive leads interconnectingthe contacts on the chip front surface with the terminals on the backingelement, these leads extending alongside the edges of the chip.Preferably, the backing element and the leads are flexible so that theterminals on the backing element are moveable with respect to the chip.Thus, the terminals desirably are moveable with respect to the contactson the front surface of the chip in directions parallel to the plane ofthe chip top and bottom surfaces. The backing element and leads providefor connection to the chip at the back surface, so that the chip can bemounted in face-up disposition on a substrate. However, because theterminals on the backing element are disposed in the central region andaligned with the chip itself, the connections to the substrate can bemade in the area beneath the chip. Therefore, the assembly need not besubstantially larger than the chip itself.

The ability to accommodate relative movement between the chip and theterminals on the backing element allows the assembly to accommodatedifferential thermal expansion between the chip and substrate.Desirably, the terminals on the backing elements are also moveablerelative to the chip in directions towards the bottom surface of thechip as discussed above, and the assembly may include resilient meansfor permitting movement of the terminals towards the bottom surface butresisting such movement. For example, the assembly may incorporate alayer of a compliant material disposed between the chip rear surface andthe terminals.

Most desirably, the assembly includes at least one generally sheet-likeflap connected to the backing element. Each such flap extends upwardly,towards the front surface of the chip and away from the backing elementalongside one edge of the chip. Each of the aforementioned leadsdesirably includes a flap portion extending along one of these flaps.The flaps may be formed integrally with the backing element. Desirably,both of the flaps and the backing element include electricallyconductive layers and a dielectric layer disposed between theelectrically conductive layers and the leads so as to provide acontrolled impedance in the leads. Assemblies of this type areespecially well suited to use with chips having contacts arranged inrows adjacent the periphery of the chip front surface peripherate.Desirably, each flap extends to the vicinity of at least one row ofcontacts. The flap portions of the leads on each such flap are connectedto the adjacent row of contacts. Such connection may be made forinstance by wire bonding or by direct connections between the flapportions of the leads and the contacts on the chip. Even where wirebonding is employed, however, the wires extending between the chipcontacts and the flap portions of the leads are short. Such short wirebonds can be readily applied and have relatively low inductance.

Most preferably, the chip assembly includes one or more support elementsdisposed between the flaps and the edges of the chip. The supportelements may cooperatively constitute a ring or box surrounding thechip. The box may also incorporate a floor element disposed beneath therear surface of the chip, between the rear surface and the backingelement. Where the assembly includes a floor element underlying the chiprear surface, the compliant layer may be disposed between the floorelement and the terminals, as, for example, between the floor elementand the backing element. These arrangements provide for mechanicalsupport of the flaps and protection of the interconnections. Furtherprotection may be afforded by encapsulating the assembly.

Further aspects of the invention provide components incorporatingsubassemblies of the backing element, leads and support element.Preferably, these components include support elements defining a box,and include flaps integral with the backing element extending upwardlyalong the sides of the box. The conductors extending along the flaps areprepositioned adjacent the top edges of the box walls. In manufacture ofthe assembly, the chip may be placed within the box and the conductorsmay be joined to the chip terminals.

Assemblies as discussed above may be incorporated in a larger assemblywith a substrate having contact pads, the contact pads of the substratebeing aligned with the terminals on the backing element and connectedthereto. Such connection may be made for example by masses ofelectrically conductive bonding material disposed between the terminalsand the contact pads of the substrate.

A further aspect of the present invention provides a circuit assemblyincluding a plurality of chip assemblies, each including an interposerand a backing element as discussed above. According to this aspect ofthe invention, the chip assemblies may be arranged in a stack, one ontop of the other, such that each chip assembly other than thebottom-most chip assembly overlies another, immediately subjacent chipassembly. The bottom surface of the backing element in each suchoverlying chip assembly faces the second surface of the interposer ofthe immediate subjacent chip assembly. Most preferably, at least some ofthe inside terminals on the backing element of each such overlying chipassembly are connected to the central terminals on the interposer of theimmediately subjacent chip assembly, so that the chips of the variouschip assemblies are electrically connected to one another.

Further aspects, features and advantages of the present invention willbe more readily apparent from the detailed description of the preferredembodiments set forth below, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic perspective view of a chip assembly inaccordance with one embodiment of the invention.

FIG. 2 is a fragmentary sectional view taken along line 2--2 in FIG. 1.

FIG. 3 is a fragmentary view, on an enlarged scale, of the areaindicated in FIG. 2.

FIG. 4 is a layout diagram depicting the spatial relationship of certaincomponents in the assembly of FIG. 1.

FIGS. 5A and 5B are fragmentary diagrammatic perspective views depictingcertain operations, in manufacture of a component utilized in theassembly of FIG. 1.

Each of FIGS. 6, 7 and 8 is a fragmentary diagrammatic perspective viewdepicting certain operations in the process of manufacture of theassembly of FIG. 1.

FIG. 9 is a fragmentary diagrammatic perspective view similar to FIG. 7but depicting components and process steps in accordance with a furtherembodiment of the invention.

Each of FIGS. 10A through 10E is a fragmentary diagrammatic perspectiveview depicting a stage in a further component fabrication processaccording to the invention.

FIG. 11 is a diagrammatic plan view of a semiconductor chip incorporatedin one embodiment of the present invention.

FIG. 12 is a view similar to FIG. 11 but showing the chip in conjunctionwith additional components.

FIG. 13 is a fragmentary, partially sectional perspective view on anenlarged scale depicting portions of the components illustrated in FIG.12.

FIG. 14 is a fragmentary, diagrammatic sectional view depicting thecomponents shown in FIG. 13 together with additional components andprocess equipment.

FIG. 15 is a fragmentary, diagrammatic sectional view depicting anassembly operation according to a further embodiment of the invention.

FIG. 16 is a fragmentary, partially sectional diagrammatic perspectiveview depicting an assembly according to a further embodiment of theinvention.

FIG. 17 is a diagrammatic plane view depicting the assembly of FIG. 16.

FIG. 18 is a diagrammatic plan view depicting an assembly according toyet another embodiment of the invention.

FIG. 19 is a fragmentary plan view depicting certain components used inthe assembly according to FIGS. 16 and 17.

FIG. 20 a fragmentary perspective view similar to FIG. 16 but depictingportions of any assembly in accordance with a further embodiment of theinvention.

FIG. 21 is a diagrammatic plan view of a component.

FIG. 22 is a fragmentary sectional view on an enlarged scale along lines22-23 in FIG. 21.

FIG. 23 is a diagrammatic perspective view of a further component usedwith the components of FIGS. 21-22.

FIG. 24 is a fragmentary sectional view taken along lines 24-24 in FIG.23.

FIG. 25 is a diagrammatic perspective view showing the components ofFIGS. 21-24 at an intermediate stage of an assembly process.

FIG. 26 is a fragmentary, partially sectional perspective view depictinga final assembly incorporating the components of FIGS. 21-25.

FIG. 27 and 28 are fragmentary, partially sectional perspective viewsdepicting components in accordance with additional embodiments of theinvention.

FIG. 29 and 30 are diagrammatic sectional views depicting still furtherembodiments.

Each of FIGS. 31, 32 and 33 is a diagrammatic, perspective viewdepicting further processes according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each chip assembly in accordance with one embodiment of the presentinvention includes a rigid substrate 20 having a top surface 22 andhaving contact pads 24 disposed on the top surface. Substrate 20 is alsoprovided with conductors 26 interconnecting certain ones of the contactpads 24. The contact pads 24 are arranged in a pattern on the topsurface of the substrate generally corresponding to the pattern ofconnections to devices, such as semiconductor chips 28 and 30 anddiscrete components 32 mounted on the substrate. Substrate 20 also hasexternal connections such as pins 34. The conductors 26 are arranged tointerconnect the various contact pads 24 in the desired patterns so asto interconnect chips 28 and 30 when the same are mounted to thesubstrate and also to connect these chips to the discrete components 32and to the external connectors 34 in the appropriate manner forfunctioning of the particular circuit. Although only a few contact pads24, conductors 26 and external connections 34 are illustrated in FIG. 1,the substrate 20 may have an unlimited number of contact pads 24,conductors 26 and external connections 34. Hundreds or thousands ofthese elements typically are provided in each substrate.

Chip 28 has a generally planar rear face 36 and a generally planar frontface 38 with electrical contacts 40 (FIG. 2) disposed thereon. Theelectrical contacts 40 are electrically connected to the internalelectronic components (not shown) of chip 28. Chip 28 is mounted onsubstrate 20 in a front-face-down orientation, with the front face 38 ofthe chip facing toward the top of face 22 of the substrate. A flexible,sheetlike dielectric interposer 42 is disposed between the chip and thesubstrate. Interposer 42 has a first generally planar face 44 facingtoward chip 28 and a second generally planar face 46 facing in theopposite direction, away from chip 28. Interposer 42 may incorporate oneor more layers. Preferably, the interposer includes a compliant,compressible layer as further discussed below. Interposer 42 has aplurality of terminals 48 on its second face 46. Each such terminal isassociated with one of the contacts 40 on chip 28 and connected to suchcontact by a flexible lead 50. Each terminal 48 is also associated withone contact pad 24 on substrate 20, and each terminal is bonded to theassociated contact pad by a mass 52 of electrically conductive bondingmaterial such as solder or a conductive polymer. Thus, the contacts onchip 40 are interconnected, via leads 50, terminals 48 and masses 52with the contact pads 24 on the substrate.

Interposer 42 has apertures 54 extending through it, from its firstsurface 44 to its second face of 46. Each aperture is aligned with onecontact 40 on chip 28. Each terminal 48 is disposed adjacent one of theapertures 54. The lead 50 associated with each terminal has a contactend 56 disposed within the associated aperture 54 and connected to theassociated contact 40 on the chip. Each lead 50 also has a terminal end58 connected to the associated terminal 48. In the structure of FIG. 2,the leads 50 are formed integrally with the terminals 48 so that theterminal end 58 of each lead merges with the associated terminal 48. Asbest seen in FIG. 2, each lead 50 is curved between its contact end 56and its terminal end 58. The curvature is in the direction perpendicularto the faces 46 and 48 of the interposer. An elastomeric, dielectricencapsulant 60 is disposed in apertures 54 so that the encapsulantcovers the contact ends 56 of leads 50 and hence covers the junctures ofthe leads with the contacts 40.

The contact end 56 of each lead 50 is moveable relative to theassociated terminal 48. As best seen in FIG. 3, the contact end 56a oflead 50a can be displaced from its normal, undeformed position (shown insolid lines) in the directions parallel to the faces 44 and 46 ofinterposer 42 and parallel to the front face 38 of chip 28. For example,the contact end 56a may be displaced to the position indicated in brokenlines at 56a'. This displacement is permitted by the flexibility of thelead 50 and by buckling and wrinkling of interposer 42. Encapsulant 60is compliant, and does not substantially resist flexing of leads 50 andbuckling and wrinkling of interposer 42. The displacement illustrated inFIG. 3, from the normal undisplaced position 56a to the displacedposition 56a' places the lead 50 in compression. That is, the terminalend 56a moves generally toward the associated terminal 48 in moving fromposition 56a to position 56a'. Movement in this direction isparticularly well accommodated by buckling of the lead 50. The contactend of each lead can also move in other directions, such as in theopposite direction from position 56a away from the associated terminal48, and in directions perpendicular to these directions, into and out ofthe plane of the drawing as seen in FIG. 3. Prefabricated leads formedon the interposer may curved in directions parallel to the face of theinterposer and parallel to the plane of the front face of the chip. Thisprovides increased flexibility in the leads. Desirably, the curvedportion of each lead overlies an aperture in the interposer. Thus, thecurved portion of the lead is not bonded to the interposer. This portionof the lead therefore can flex to accommodate relative movement of thecontact and terminal without deformation of the interposer.

As best seen in FIG. 4, the contacts 40 on chip 28 (each symbolized by adot in FIG. 4) are disposed in a pattern on the front surface of chip28. Contacts 40 cooperatively encompass a contact pattern area 62 on thefront face of chip 28. The boundary of the contact pattern area isillustrated by a broken line B in FIG. 4. The boundary of the contactpattern area may be taken as the shortest combination of imaginary linesegments along the front face of the chip which cooperatively encloseall of the contacts 40. In the particular example illustrated in FIG. 4,this boundary is generally in the form of a rectangle. Contacts 40 aredisposed throughout contact pattern area 62', in locations determined bythe interior structure of chip 28. Contact pattern area 62 includes aperipheral region, adjacent the boundary B, and a central region,adjacent the geometric center 64 of the contact pattern area. Contacts40 are disposed both in the peripheral region and in the central region.Typically, although not necessarily, the contacts 40 are disposed atsubstantially equal spacings throughout the entirety of contact patternarea 62. The terminals 48, each symbolized by an X in FIG. 4, aredisposed in a similar pattern on the second surface 46 of interposer 42.At least some of terminals 40 are disposed in the area of interposersurface 46 overlying contact pattern area 62. Terminals 64 encompass aterminal pattern area 66 on the second face 46 of the interposer. Theboundary of terminal pattern area 66 is illustrated in FIG. 4 by thebroken line T. The boundary of the terminal pattern area may be taken asthe shortest combination of imaginary line segments which wouldcooperatively enclose all of the terminals on the second surface of theinterposer. The geometric center of terminal array area 66 desirably iscoincident, or approximately coincident, with the geometric center 64 ofthe contact array area. Desirably, terminal pattern area 66 is notsubstantially larger than contact pattern area 62. That is, theperimeter of the terminal area preferably is less than about 1.2 times,and most preferably about 1.0 times the perimeter of contact patternarea 62. Stated another way, the outermost terminals 48 desirably liewithin or close to the boundary B of contact array area 62. The totalarea encompassed within terminal pattern area 66 desirably is less thanabout 1.4 times, and most desirably about 1.0 times the total areaencompassed within contact pattern area 62. Thus the leads 50 connectingcontacts 48 to terminals 40 do not "fan out", away from the geometriccenter 64 the contact pattern area. Typically, the mean distance of theterminals 48 from geometric center 64 of the contact pattern area,measured in the direction parallel to the surfaces of the chip andinterposer, is less than about 1.1, and typically about 1.0, times themean distance of the chip contacts 40 from center 64.

The interposer and leads utilized in the structure of FIGS. 1-4 may befabricated by a process as schematically illustrated in FIGS. 5A-5B. Inthis procedure, the terminals 48 and leads 50 may be deposited on thesecond surface 46 of the sheetlike interposer by conventional printedcircuit manufacturing techniques before formation of apertures 54. Thus,the leads and terminals may be formed either by an additive process,wherein the metal is deposited in the desired pattern by plating, orelse in a subtractive process which begins with a laminate includingboth the sheetlike interposer 42 and a full layer of metal and removesthe metal except in the areas where the terminals and leads are desired,so as to yield a sheet having the terminals and leads in position (FIG.5A). After formation of the terminals and leads, apertures 54 are formedin registration with the contact ends 56 of the leads 50 (FIG. 5B) byetching through the interposer from the first surface 44, or by applyingradiant energy such as laser beam focused at the appropriate spots onthe first surface 44.

A further method of making a component incorporating the interposer,terminals and leads is shown in FIGS. 10A-10E. In this method, theapertures 54 are formed in interposer 42, and the aperture interposer isprovided with a layer 302 of adhesive on the second surface 46 of theinterposer. A conductive sheet, such as a sheet of copper 304 is appliedon the first surface of the interposer, so that sheet 304 overliesadhesive 302 and so that sheet 304 overlies the apertures 54. A firstsurface 306 of sheet 304 faces towards interposer 42 and confronts thesecond surface 46 of the interposer, with the adhesive layer 302disposed there between. A second surface 308 of the conductive sheetfaces away from the interposer. A layer 310 of a photosensitive resistcomposition is applied on the second surface 308 of conductive layer304. A second resist composition 312 is placed within apertures 54 sothat resist 312 covers the first surface 306 of conductive layer 304within apertures 54. Desirably, resist 312 is applied by applying alayer of the second resist composition to the first surface 44 ofinterposer 42 as illustrated in FIG. 10B. Both resist compositions 310and 312 may be provided as so-called "dry resist" i.e., as a film ofresist composition which can be laminated to the other structures.Resist composition 312 is laminated to the first surface 44 of theinterposer 42 under pressure so that the resist composition flows intoapertures 54 and substantially fills these apertures.

In the next stage of the process, depicted in FIG. 10C, the first resistlayer 310 is selectively cured and uncured portions are removed so as toleave the cured resist in a pattern corresponding to the desired patternof conductive materials in the finished product. Such selective curingand removal of a resist layer may be accomplished by known photographictechniques. The remaining resist pattern on the second surface 308 ofthe conductive layer 304 includes elongated lead areas 314 and terminalareas 316 contiguous with the lead areas. At least a part of each leadarea 314 overlies one of the apertures 54 in the interposer, whereas theterminal areas 316 do not overly the apertures. The portion of each leadarea 314 overlying an aperture is smaller than the aperture, so thateach lead area overlies only a portion of the associated aperture 54.Desirably, each lead area 54 protrudes lengthwise across the aperture54, as illustrated in FIG. 10C. The second resist material 312 withinapertures 54 desirably also is cured. As the second resist material maybe cured in its entirety, and need not be cured selectively in apredetermined pattern, the second resist material may be of a type whichcan be cured by exposure to heat or other nonselective curing method.Alternatively, the second resist material 312 may be photographicallycured.

In the next stage of the process, illustrated in FIG. 10D, the assemblyis immersed in an etchant capable of dissolving the conductive materialin layer 304 so that the etchant contacts this layer. During the etchingprocedure, the first resist in lead area 314 and terminal areas 316protects the second surface 308 of conductive layer 304. The interposer42 protects the first surface 306 of layer 304 in the terminal areas 316and in those portions of lead areas 314 which do not overly apertures54. The second resist 312 protects the first surface 306 in thoseportions of lead areas 314 which overlie apertures 54. The etchanttherefore does not attack those portions of conductive layer 304 coveredby lead portions 314 and terminal portions 316 of the first resist layer310. The first resist layer 310 and the second resist 312 are thenremoved by conventional resist decomposition processes such as exposureto solvents which attack the resist. This leaves the unattacked portionsof conductive layer 304 as leads 50 and terminals 48 on the secondsurface 46 of interposer 42, with a contact end 56 of each lead 50protruding over the associated aperture 54 and with a terminal end 58 ofeach lead connected to the associated terminal 48.

This process can be modified. For example, the adhesive layer 302 may beomitted where the conductive layer forms a satisfactory bond to thematerial of the interposer. Also, the pattern first resist 310 need notbe provided by a subtractive process as discussed above but instead maybe provided by an additive process, wherein the resist is applied onlyin the areas to form the pattern, as by silkscreening. Formation of theleads 50 and terminal 48 by this type of etching process is particularlyuseful in forming fine leads in good registration with apertures 54.Also, as the apertures 54 are pre-formed, there is no possibility ofdamaging the leads during formation of the apertures.

The assembly of the interposer and terminals and contacts is fabricatedin a substantially continuous sheet or strip. As illustrated in FIG. 6,the interposers may be provided in the form of a continuous tape 70,with plural interposers 42 spaced lengthwise along the tape, each suchinterposer having terminals 48 and leads 50 thereon. Tape 70 may be inthe form of a single sheet of the material employed for the interposers42, or else may include separate pieces of such material, eachconstituting one or more interposers, secured to a backing or the like.Tape 70 may have sprocket holes (not shown) or other features such asthose commonly utilized on the tapes for tape automated bonding ofsemiconductor chips.

In an assembly method according to the invention, tape 70 is advanced ina downstream direction (to the right as seen in FIG. 6) and chips 28 areconnected to the tape upon assembly of each chip with one interposer 42and with the associated terminals and leads. The chips are subsequentlycarried downstream with the tape, through further operations asdiscussed below.

As best seen in FIG. 7, each interposer, with the terminals 48 and leads50 thereon, is brought into juxtaposition with a chip 28, and the chipis aligned with the interposer so that each aperture 54 is aligned withone contact 40 of the chip. The interposer 42 and chip 28 are broughttogether, so that the first face 44 of the interposer bears on the frontface 38 of the chip, and the contacts are received in the apertures 54of the interposer. The contact end 56 of each lead 50 initially liessubstantially in the plane of the second surface 46 of the interposer. Atool 74 is advanced into engagement with the contact end 56 of each leadso as to deform the contact end 56 downwardly, into the underlyingaperture 54 and towards the associated contact 40. Tool 74 may be asubstantially conventional thermal bonding tool, thermosonic bondingtool, ultrasonic bonding tool, compression bonding tool, or the like ofthe types commonly used in tape automated bonding or wire bonding. Byadvancing the tool 74 into each aperture 54, the contact ends of leadsare manipulated within the apertures and bonded to the contacts 40 onthe chip. Although only a single tool 74 is depicted in FIG. 7, thebonding operation may be performed in a multiple operation, with many orall of the leads 50 being bonded to the associated contacts at once.

After the contacts and leads have been bonded to one another, theinterposer and the chip are advanced to a further station, where theencapsulant 60 is applied within each aperture 54. The encapsulant 60may be applied dropwise, by conventional drop application equipment. Asbest seen in FIG. 8, each drop of encapsulant 60 covers the contact end56 of the associated lead, but leaves the associated contact 48uncovered. The encapsulant protects the relatively delicate contact ends56 of the leads and the relatively delicate junctures with the terminals40. Once the encapsulant has been applied, the assembly of theinterposer, leads, terminals and chips is advanced to a testing station.As illustrated in FIG. 8, the assembly, including the chip 28, may betested. The test may involve connection of the chip, through theterminals 48, to an external electronic test device (not shown). Thetest device may be arranged to operate the chip under power for anappreciable period so as to "burn-" the chip and detect any latentdefects. Typically, numerous connections should be established to thechip simultaneously. As illustrated in FIG. 8, this may be accomplishedby applying probes 76 to terminals 48. Probes 76 may be so called"noncompliant" probe That is, probes may be arranged to move in unison,in the directions towards and away from the chip 28 (upwardly anddownwardly as seen in FIG. 8). The probes 76 are mounted to a commonfixture (not shown) so that the vertical position of the probes relativeto one another are fixed. This type of "noncompliant" probe array isparticularly convenient where the required spacings between probes (thespacings of-the terminals 48) are relatively small. However,non-uniformities in the dimensions of the probes 76 and/or in thedimensions of the terminals 48 or chip 28 may cause one or more of theprobes 76 to engage the associated terminal 48 before the other probeshave engaged their terminals. Desirably, interposer 42 is compliant, sothat each terminal 48 can be displaced slightly by the associated probe76 in the direction toward chip 28. The region of interposer 42 beneatheach terminal 48 compresses slightly to accommodate such displacement.This allows all of the probes 76 to engage their associated contacts 48without imposing excessive loading on any one probe. The terminals 48may be larger than the contacts on the chip, so as to provide arelatively large area for engagement by each contact 76 and thusaccommodate a reasonable amount of misalignment of the contacts in thedirections parallel to the faces of the interposer. Because each chipcan be tested in this fashion, prior to assembly with the substrate,defects in the chips, in the terminals and leads associated with theinterposer and in the bonds between the leads and the chip contacts canbe detected before the chip is united with the substrate.

After the testing operation, the chip and interposer are united with thesubstrate. The chip and interposer assembly is oriented so that thesecond face of the interposer, and the terminals 48, face the topsurface of the substrate, and each terminal 48 confronts one contact pad24 on substrate. Masses of solder are applied between the confrontingterminals 48 and contact pads 24 and melted in a "solder reflow"operation so that the solder forms a solid joint between the contact padand terminal, and so that the solder masses support the chip andinterposer assembly above the substrate 20, in the orientationillustrated in FIG. 2. The solder application and reflow operation maybe performed in substantially the safe way as the solder application andreflow operation of conventional flip-chip bonding. Thus, the masses ofsolder may initially be applied to the contact pads 24 of the substrate,before the chip and interposer assembly is united with the substrate.Alternatively, the solder may be applied to the terminals 48 and bondedto the contact pads 24 in the reflow operation. A flux typically isemployed in the solder reflow operation. Because the solder massessupport the chip and interposer surface assembly above the substrate,there is a gap 80 between the interposer and the substrate. Fluxresidues may be rinsed out of the assembly by passing a rinsing fluidthrough this gap.

In an assembly method according to a further embodiment of theinvention, the interposer 42 is not provided with leads before theinterposer is united with the chip 28. Instead, leads 50' are applied bybonding separately formed pieces of fine wire to the terminals 48 and tothe contacts 40 after the interposer is assembled with the chip. Leads50' are flexible and curved, and arranged to deform as discussed aboveso that each contact 40, and the associated contact end of the lead 50'can move relative to the associated terminal 48 so as to accommodatethermal expansion. In the embodiment illustrated in FIG. 9, a layer ofan adhesive 81 is disposed between the first surface of the interposerand the front surface of the chip.

The subassembly illustrated in FIG. 9 may be further provided with anencapsulant (not shown) in the form of a layer covering substantiallythe entire second face 46 of interposer 42 and hence filling theapertures 54 and covering the leads 50'. The layer is provided withholes in alignment with the terminals 48. These holes may be formed byetching the encapsulant layer by applying this layer in a selectivecoating process such as silk screening or the like or by applying theencapsulant layer in a selective curing process. Thus, the encapsulantwhich may be curable by ultraviolet or other radiant energy. Theencapsulant may be deposited over the entire interposer, and overterminals 48. After application of the encapsulant, radiant energy maybe applied selectively, so that the areas of the layer overlyingterminals 48 remain uncured. These layers are then removed by washing orby a relatively mild etching operation, leaving holes in alignment withterminals 48. Alternatively, the encapsulant layer may be curednonselectively and then portions may be removed by applying radiantenergy such as laser light in alignment with terminals 48. Masses ofelectrically conductive bonding material are deposited within theseholes in the encapsulant layer. These masses are then engaged with thecontact pads (not shown) of the substrate and heated so that bondingmaterial forms a bond between each terminal 48 and the associatedcontact pad on the substrate, in a manner similar to the solder bonds ofthe assembly depicted in FIG. 2.

A chip may have contacts disposed in a peripheral arrangement, i.e.,where all of the contacts are disposed adjacent the periphery of thechip and hence adjacent the periphery of the contact pattern area. Thecentral zone of the contact pattern area, adjacent the geometric centerof the contact array, may be devoid of contacts. With such a chip, theterminals on the interposer may be arranged in a "fan in" pattern, i.e.,where the mean distance from the geometric center of the contact arrayto the terminals on the interposer is less than the mean distance fromthis geometric center to the contacts on the chip. Some of the terminalsare disposed on the area of the interposer overlying the central,contact-free zone of the contact pattern area. This arrangement canprovide a substantially uniform distribution of terminals over an areaequal to the contact pattern area. This provides a spacing betweenadjacent terminals larger than the spacing between adjacent contacts.Such an arrangement allows connection of chips with peripheral contactarrays to area arrays of contact pads on the substrate. Thus chipsoriginally intended for conventional bonding processes such as tapeautomated bonding can be adapted readily and economically to substrateshaving compact contact pad arrays similar to those used in flip-chipbonding.

As illustrated in FIG. 31, chips 928 may be provided in the form of awafer 930 incorporating a plurality of chips, all of the same design orof differing designs. Individual, separate, interposers 942 may bepositioned on the individual chips constituting wafer 930 and theinterposers may be assembled to the chips as discussed above. In thisoperation, the contacts on each chip 928 are secured to the leads andterminals of each interposer. After the interposers are secured to thechips, and desirably after the junctures between the leads of eachinterposer and the contacts of each chip are encapsulated, theindividual chips are separated from the wafer and from one another, asby cutting the wafer using conventional wafer severing or "dicing"equipment commonly utilized to sever individual chips withoutinterposers. This procedure yields a plurality of chip and interposersubassemblies, each of which may be secured to an individual substrate.

Alternatively, as illustrated in FIG. 32, a wafer 950 incorporating aplurality of chips may be assembled to a sheet 952 incorporating aplurality of interposers 954. Again, the contacts on each chip aresecured to the terminals and leads of one individual interposeroverlying the particular chip. The wafer 950 and the sheet 952 aresevered after this operation, and desirably after encapsulating theleads, so as to provide individual subassemblies each including a chipand an interposer.

Interposers also may be provided in the form of a sheet 960incorporating plural interposers such as interposer 962 and 964 atpredetermined relative positions corresponding to the positions of chipson a completed assembly including a substrate. Chips 966 and 968 may besecured to the individual interposers and the entire assembly of pluralchips and the sheet of plural interposers may be secured to a substrate970. Each interposer in such an assembly desirably incorporates apattern of terminals and leads as discussed above. This variant of theassembly procedures provides for consolidation of plural chips into alarger subassembly before bonding to the substrate.

A semiconductor chip 820 used in a further embodiment of the inventionhas a generally planar front face 822 (the face visible in FIG. 11)having a central region 824 adjacent the geometric center of the faceand a peripheral region 826 adjacent the edges 828 bounding face 822.The front or contact-bearing face 822 of the chip is regarded asdefining the top of the chip. Thus, in specifying directions, thedirection pointing out of front face 822, and away from the chip, i.e.,the direction pointing out of the plane of the drawing towards theviewer in FIG. 11, is the upwardly direction. The downward direction isthe opposite direction. As used in the present disclosure with respectto a semiconductor chip assembly, such terms should be understood asbased on this convention, and should not be understood as implying anyparticular directions with respect to the ordinary gravitational frameof reference. The chip 820 also has a plurality of peripheral contacts830 arranged in rows 832, there being one such row adjacent each edge828 of the chip. The rows 832 do not intersect one another but insteadterminate at appreciable distances from the corners of the chip so thatthe corners 834 are devoid of peripheral contacts 830. The centralregion 824 of the chip front surface 822 is also devoid of contacts. Thecontacts 830 in each row 832 are spaced at very close intervals,typically about 100 to about 250 micrometers center to center. Thiscenter to center spacing is adequate for wire bonding or tape automatedbonding. This chip configuration is typical of high I/O count chipsoriginally intended for use with wire bonding or tape automated bondingsystems.

In an assembly method according to one embodiment of the invention, asheet-like dielectric interposer 836 is assembled to chip 820.Interposer 836 includes a flexible top layer 838 (FIG. 13) formed by athin sheet of material having a relatively high elastic modulus and acompliant bottom layer 840 formed from a material having a relativelylow elastic modulus. The high-modulus material of top layer 838 may be apolymer such as a polyimide or other thermoset polymer, a fluoropolymeror a thermoplastic polymer. The compliant, low-modulus material ofbottom layer 840 may be an elastomer. Desirably, the low-modulusmaterial has elastic properties (including modulus of elasticity)comparable to those of soft rubber, about 20 to 70 Shore A durometerhardness. Interposer 836 has a first or bottom surface 842 defined bybottom layer 840 and a second or top surface 844 defined by top layer838. Bottom, compliant layer 840 includes holes or voids 841interspersed with masses 843 of the low-modulus material.

Interposer 836 has edges 846 bounding surfaces 842 and 844 and extendingtherebetween. The interposer also has a plurality of central terminals848 distributed over the second or top surface 844. Terminals 848 aredisposed at substantially even spaces on surface 844 so that terminals848 constitute a "area array". The dimensions of interposer 836 in theplane of top surface 844 are smaller than the corresponding dimensionsof chip 820 in the plane of front surface 822. The number of centralterminals 848 may be approximately equal to the number of peripheralcontacts 830 on the semiconductor chip Nonetheless, the center-to-centerlinear distance between adjacent ones of central terminals 848 issubstantially greater than the center-to-center distance betweenadjacent peripheral contacts 830 on the chip, because the centralcontacts 848 are substantially evenly distributed rather thanconcentrated in only a few rows. Each central terminal 848 is alignedwith one of the masses 843 of low-modulus material in compliant layer840, whereas the holes 841 in the complaint layer are out of alignmentwith the central terminals 848. In a variation of this embodiment, theholes may be aligned with terminals 848. In a further variation, theholes may be continuous with one another whereas the masses oflow-modulus material may be separate posts or pillars entirelysurrounded by such continuous holes.

As best seen in FIG. 13, each central terminal 848 is connected with apartial lead 50 and a bonding terminal 852 which are formed integrallywith the central terminal. Central terminals 848, partial leads 50 andbonding terminals 852 may be formed from substantially any electricallyconductive material, but preferably are formed from metallic materialsuch as copper and copper alloys, noble metals and noble metal alloys.These components typically are fabricated on the top or second surface844 of interposer 836 by conventional photolithographic end etching ordeposition techniques. Bonding terminals 852 are arranged in rows 54adjacent the edges 846 of the interposer. As best seen in FIG. 12, thereare four such rows 54 of bonding terminals, one adjacent each edge ofthe interposer.

In the assembly method according to this embodiment of the invention,the interposer 836 with the preformed terminals 848, partial leads 50and bonding terminals 852 thereon is positioned on chip 820 so that thefirst surface 842 of the interposer faces the front surface 822 of thechip, and so that the edges 846 of the interposer are disposed inwardlyof the rows 832 of peripheral contacts 830 on the chip. Bondingterminals 852 are electrically connected to contacts 830 on the chip bya conventional wire bonding operation. The arrangement of the bondingterminals 852 in rows parallel to and adjacent to the rows of peripheralcontacts 830 on the chip substantially facilitates the wire bondingprocess. The fine, flexible bonding wires 856 applied in the wirebonding operation merge with the bonding terminals 852 and partial leads50 on the interposer to form composite leads extending from theperipheral contacts of the chip to the central terminals on theinterposer. As best appreciated with reference to FIG. 13, each suchcomposite lead extends inwardly from one peripheral contact 830 to anassociated central terminal 848 in the central way. Each such compositelead extends across the edge 846 of the interposer.

In the next stage of the process, a low elastic modulus dielectricencapsulant or solder masking material such as a silicone rubber orother castable elastomer 858 (FIG. 14) is applied over the interposerand chip and over bonding wires 856. The encapsulant is applied so as toleave holes 860 in alignment with each of the central terminals 848 onthe interposer. This may be accomplished as discussed above withreference to the assembly of FIG. 9. At this stage, the assembly isrelatively rugged and can be handled readily. Thus, the wires 856 arefully protected by the encapsulant.

Either before or after the encapsulant 858 is applied, the chip and allof the connections made within the assembly can be tested by makingtemporary electrical connections to the central terminals 848. Becausethe central terminals 848 are at substantial center-to-center distances,they may be readily contacted with probes such as the plural probe set862 schematically illustrated in FIG. 14. Moreover, because the bottomlayer 840 of the interposer is compliant, each central terminal 848 isdisplaceable towards and away from the front surface 822 of the chip820. Thus, the bottom layer can be compressed by the tips 864 of theprobe set 862. This greatly facilitates making good electrical contactbetween a plurality of probes and a plurality of central terminals atonce, and hence greatly facilitates electrical testing of the chip andthe other components of the assembly. The configuration of compliantlayer 840 contributes to this action. Each mass 843 of low-modulusmaterial provides backing and support for the aligned terminal 848. Asthe tips 864 of the test probe set 862 engage the terminals, each mass843 is compressed in the vertical direction and therefore tends to bulgein horizontal directions, parallel to the plane of the chip. Holes 841provide space for such bulging. Each terminal 848 can move downwardlytoward the chip substantially independently of the other terminals.Compliant layer 840 need only provide for sufficient downward movementof terminals 48 to accommodate tolerances in the components and testequipment by accommodating differences in vertical position betweenadjacent terminals and/or test probes. Typically, about 0.125 mm or lesscompliance is sufficient. For example, complaint layer 840 may be about0.2 mm thick.

Although test probe set 862 is schematically illustrated as includingonly a few tips 864, the test probe set in fact may include a fullcomplement of tips 864, equal in number to the number of terminals 848,so that all of terminals 848 can be engaged simultaneously. The tips ofprobe set 862 may be rigidly mounted to a common support 865. Therefore,the test probe set may be rugged, reliable and durable. The particularshape of tips 864 is not critical. However, tips 864 may desirably beformed as small metallic spheres solder-bonded to support 865. Support865 in turn may be a ceramic body with appropriate internal leads,similar to a conventional semiconductor substrate. Because the testprobe set may make simultaneous connections with all terminals in thesubassembly, and because the test probe set may have dimensions andconfiguration similar to a real substrate, the temporary electricalconnection made using the test probe can provide a realistic test of thechip and interposer subassembly. In particular, the test probe set neednot involve long leads which may introduce unwanted inductance and/orcapitance. Accordingly, the test probe set can be employed to test andoperate the chip at full speed. Because the test probe set may be asimple, economical device, many such probe sets can be provided in amanufacturing plant, so that each chip can be tested for a prolongedperiod.

In the next stage of the assembly operation after testing, the chip andinterposer subassembly is juxtaposed with a substrate having electricalcontact pads thereon. The assembly is placed on the substrate so thatthe central terminals 848 face toward the electrical contact pads on thesubstrate, and so that each central terminal 848 is aligned with onecontact pad. Masses of an electrically conductive bonding material suchas a solder or an electrically conducted adhesive may be disposedbetween the central terminals and the contact pads of the substrate.These masses may then be caused to flow and to bond with the centralterminals 848 and the contact pads thereby forming mechanical andelectrical connections between the central terminals and the contactpads. This stage of the process may utilize essentially the sametechniques as are employed in surface mount technology for assembly ofcomponents on printed circuit boards. Because the central terminals 848are disposed at substantial center-to-center distances, the standardsurface mount techniques can be used without difficulty. For example, ahigh I/O count can be achieved with 10-25 mil (250-625 micrometer)center-to-center distances. In an alternate embodiment, each contact padon the substrate may be a microminiature separable connector such as asocket, and a mating separable connector may be provided on eachterminal. For example, each terminal 848 may incorporate a miniature pinadapted to engage such a socket. In this case, the pins would serve asthe means for connecting terminals 848 to the contact pads of thesubstrate. The encapsulent or solder mask layer can be provided withmetal rings surrounding each hole 860 and hence surrounding eachterminal 848. Each such ring defines a preselected area which can bewetted by solder and thus confines the solder of each joint to apreselected area. Also, small studs, balls, or pins may be positioned inthe holes of the solder mask layer in electrical contact with theterminals 848, and these studs may be soldered to a substrate.

Inasmuch as each peripheral contact 830 on the chip is connected to oneof the central terminals 848 on the interposer, and each such centralterminal is connected to one of the contact pads on the substrate, eachperipheral contact 830 is connected to one of the contact pads of thesubstrate. The substrate contact pad of course may be connected to otherelements of an electrical circuit through conventional connections (notshown) incorporated in the substrate. For example, substrate may be acircuit board, circuit panel or hybrid circuit substrate incorporatingvarious electronic elements in addition to chip 820.

The interconnections between the chip and the substrate (betweenperipheral contacts 830 and contact pads) are accommodated within thearea of the chip itself, i.e., within the area on the substrate occupiedby chip 820. Thus, no space on the surface of the substrate is wasted bya conventional "fan-out" pattern of interconnections. Moreover, theassembly is substantially resistant to thermal cycling. Each of thecomposite leads connecting one of the chip peripheral contacts and oneof the central terminals 848 on the interposer is flexible. Thus, thepartial leads 50 (FIG. 13) on the interposer surface itself preferablyare flexible, and the fine bonding wires 856 are also flexible. Theinterposer itself, and particularly the top layer 838 and bottomcompliant layer 840 may be flexible. Accordingly, there can besubstantial movement of terminals 848 on the interposer relative tocontacts 830 on the chip in directions parallel to the chip frontsurface. Such movement can be accommodated without applying substantialforces to the junctions between the leads and the chip contacts. Duringuse of the assembly, differential thermal expansion of chip 820 andsubstrate may cause appreciable displacement of the contact pads on thesubstrate relative to peripheral contacts 830 on the chip. Inasmuch asthe central terminals 848 of the interposer are bonded to the contactpads of the substrate by relatively stiff noncompliant conductivemasses, the central terminals will tend to move with the contact pads.However, such movement is readily accommodated and does not result insubstantial stresses at the bonds between the central terminals andcontact pads.

The assembly shown in FIG. 15 has an interposer 836' similar to theinterposer discussed above with reference to FIGS. 11-14. However, theprefabricated leads 850' associated with terminals 848' have outer orcontact portions 854' projecting outwardly beyond the edge 846' of theinterposer. As prefabricated leads 850' are disposed on top layer 838'of the interposer, the prefabricated leads cross the edge 846' of theinterposer at an appreciable height above the first or bottom surface842' of the interposer. The projecting outer portions 854' are curveddownwardly, toward the first surface 842' of the interposer. Thiscurvature desirably is provided during fabrication of the interposer andleads, before the interposer is assembled to the chip. In the assemblyoperation, the interposer 836', with the leads 850' and terminals 848'already mounted thereon is placed onto chip 820' so that the outerportions 854' are in alignment with contacts 830' of the chip. Thecurvature of the leads places the outer or contact portions 854' inclose proximity to chip contacts 830' A tool 855 is then applied to theouter portions 854' so as to force the outer portions thus forcing leads854' into engagement with the chip contacts 830' so as to bond the outerportions 854 of leads 850' directly to the chip contacts. Typically,pressure is applied through tool 855 along with heat and/or ultrasonicenergy. This stage of the process may employ conventionalthermocompression or ultrasonic bonding techniques commonly used to bondinner leads in a tape automated bonding or "TAB" operation. This bondingestablishes a connection between each chip contact 850' and one of theterminals 848' on the interposer, without the need for any intermediatewire bonding operation. Once the contacts and terminals are connected inthis manner, the resulting subassembly can be encapsulated and bonded toa substrate in substantially the same fashion as discussed above. Asleads 850' are flexible, terminals 848' are movable with respect tocontacts 830' to compensate for thermal expansion.

The terminals 848' and leads 850' used in this structure can befabricated by photolithigraphic techniques. For example, the interposermay initially be fabricated with a solid sheet of copper or other metalcovering the second surface 844' and extending beyond edges 846'. Theseportions of the metal sheet extending beyond the edges of the interposermay be embossed to impact a downward curvature. The surface of themetallic layer facing upwardly away from the interposer (facing towardthe top of the drawing in FIG. 5) may be covered with a conventionalphotoresist pattern such that the photoresist covers the areascorresponding to the terminals 848' and leads 850'. The opposite surfaceof the sheet may be covered with a further photo resist in the areasextending beyond the edges 846' of the interposer. The sheet may then beexposed to an etching solution so as to remove those areas not coveredby the photo resist on the top surface, i.e., to remove all areas of themetal sheet other than the terminals 848' and leads 850'. The photoresist may be removed, leaving interposer with the terminals and leadsthereon. The curvature imparted to the metal sheet by embossing providesthe desired downward curvature in the outer portions 854' of the leads.Alternatively, the leads may be bent after etching, using a forming die.In yet another lead-forming method, the dielectric interposer, or one ofthe generally planar dielectric layers constituting the interposer maybe provided with features projecting out of the plane of the layers,such as bumps or elongated ridges. The leads may be formed by depositingmetal or other conductive material so that it forms leads extending overthe projecting features and then removing those portions of thedielectric layer or interposer constituting the projecting features, asby selectively etching the dielectric layer, leaving behind leads whichare curved out of the plane. The step of depositing the conductivematerial to form the leads may be performed by selectively depositingthe conductive material using conventional techniques, or by depositingconductive material and selectively etching or otherwise removingconductive material before etching the dielectric layer.

An alternate, generally similar arrangement, includes an interposerincorporates a flexible top layer similar to the top layer 838 of theinterposer discussed above with reference to FIGS. 11-14. Terminals andleads are positioned on the first or bottom surface of this layer, sothat the terminals face towards the chip when the layer is in positionon the chip. The interposer may also include a separate compliantunderlayer disposed between the top layer and the chip front surface,and also disposed beneath terminals i.e., between the terminals and thechip. The compliant layer may be positioned on the chip surface, beforethe top layer, and terminals are positioned on the compliant layer. Inthis case, the compliant layer may incorporate adhesives at its top andbottom surfaces so as to bind the top layer to the chip. Because thecompliant layer is soft, the top layer will remain flexible even whenbound to the chip through the compliant layer, and the terminals willstill be movable with respect to the contacts in directional parallel tothe face of the chip. Alternatively, the compliant layer may be formedfrom a partially cured elastomer such as a so-called "B-stage" siliconeelastomer. After assembly of the top layer, this partially curedmaterial may be more fully cured, as by heating it, which causes theelastomer to bond with the top layer and with the chip surface. In thisarrangement, the terminals are disposed beneath the top layer. Toprovide access to the terminals from the second or top surface of theinterposer, the interposer top layer is punctured as applying radiantenergy from a radiant energy source such as a laser in registration withthe terminals to thereby form holes in alignment with the terminals.Once the holes have been formed, the resulting subassembly can be bondedto a substrate in the same manner as discussed above. These holes may beformed before the interposer is connected to the chip, and indeed may beformed before the terminals are positioned on the interposer. In afurther alternative arrangement, the terminals and leads can be providedon the compliant layer itself.

The assembly illustrated in FIG. 16 is similar to the assembly of FIG.15. However, the outboard portions 8354 of leads 8350 have outwardextensions projecting outwardly beyond chip peripheral contacts 8330.These outward extensions are secured to a securement element 8361.Although only one securement element 8361 is visible in FIG. 16, itshould be clearly appreciated that a similar securement element 8361 isprovided at each edge of interposer 8336 as seen in FIG. 17. Eachsecurement element serves to reinforce and support the outboard portionsof the leads, and to prevent undesired bending of the leads indirections parallel to the surfaces of the interposer and chip duringassembly. The central terminals 8348 and peripheral contact leads 8350associated with interposer 8336 are disposed on the first or chip-facingsurface 8342 of the interposer top layer 8338. As best seen in FIG. 17,the securement elements 8361 are connected to interposer 8336 by bridgeelements 8363. The bridge elements are disposed at spaced-apartlocations around the periphery of the interposer. Preferably, theinterposer, securement elements and bridge elements are formed as anintegral unit. All of these components may be portions of a unitarysheet of dielectric material. Thus, the interposer 8336, bridge elements8363 and securement elements 8361 may all be formed as part of anelongated tape 8381,(FIG. 17) which may include several interposers8336, each with its associated securement elements and bridge elements.The tape may also include waste or trim areas 8383. During the variousassembly and handling operations, the interposers and chips may beadvanced through the process by advancing the tape.

Bridge elements 8363 are disposed at the comers of the interposer. Thechip 8320 used in this assembly includes four rows 8332 of peripheralcontacts 8330, the rows forming a generally rectangular pattern.However, the rows of peripheral contacts stop short of the comers ofthis rectangular pattern, so that the corner regions of the pattern aresubstantially devoid of contacts 8330. Bridge elements 8363 overliethese corner regions, and hence do not cover any of the contacts 8330.

Each securement element 8361 includes a top layer 8301 (FIG. 16). Eachsecurement element has an inboard edge 8365 extending generally parallelto an edge 8346 of interposer so that these parallel edges define anelongated slot 8367 between the securement element and the interposer.Slots 8367 are aligned with the rows 8332 of chip peripheral contacts8330. The peripheral contact leads 8350 extend across slots 8367, theoutward extensions 8354 of these leads being attached to the securementelements 8361, so that each peripheral contact lead 8350 is supportedboth by the interposer and by the securement element.

Each securement element 8361 has a single row of outside terminals 8372extending generally parallel to the adjacent slot 8367. Outsideterminals 8372 are disposed on the first or chip-facing surface 8369 ofthe top layer 8301 of each securement element 8361. Outside terminalleads 8374 (FIG. 16) extend inwardly from outside terminals 8372 acrossslots 8367. Each such outside terminal lead has an inboard end 8376secured to the interposer 8336. Thus, both the outside terminal leads8372 and peripheral contact leads 8350 extend across slot 8367. Theseleads are interspersed with one another along the length of each slot8367.

Holes 8360 are provided in the interposer and in each securement elementtop layer in alignment with the central terminals 8348 and outsideterminals 8372 so that the central terminals and outside terminals areaccessible from the second surfaces of the interposer and securementelements, i.e., from the surface facing away from the chip.

Interposer 8336 includes a compliant bottom layer 8340, and eachsecurement element 8361 may include a compliant bottom layer 8303 (FIG.16). All of these compliant layers may be similar to the compliantlayers discussed above, and may include holes (not shown) to increasetheir compliance. The compliant layers of the interposer and securementelements may be formed and assembled separately from these components,or may be incorporated in tape 8381.

The leads and terminals may be formed in place on the interposer and onthe securement elements by an etching process similar to those describedabove. A copper or other metallic sheet may be laminated to thedielectric sheet which will ultimately form the interposer top layer8338 and the securement element top layers 8301, and then covered with aphotoresist pattern and etched to form the various terminals and leads.Holes 8360 and slots 8367 may be formed after the terminals and leads,by selectively applying radiant energy such as laser radiation to thesheet to selectively remove portions of the sheet. Alternatively, theslots and holes may be formed before the leads and terminals, as byetching or mechanically punching the dielectric sheet. The leads andterminals may then be formed by applying and selectively etching ametallic layer. In this case, the holes and slots in the dielectricsheet should be temporarily filled with a resist to prevent unwantedetching of the leads and terminals by etchant entering through the holesand slots. Peripheral contact leads 8350 and outside terminal leads 8374are bent downwardly, toward the bottom of the interposer, within slots8367. The downward curvature of these leads may be formed by embossingthe sheet used to fabricate these leads. Thus, although each lead 8350and 8374 extends into a slot 8367 from above the bottom layers 83083 and340 of the securement elements and interposer, each such lead extends tothe bottom of the interposer. Before the interposer is assembled to thechip, a set of support elements 8307 is juxtaposed with chip 8320 sothat one such support element lies alongside each edge 8309 of the chip.As best seen in FIG. 19, support elements 8307 may be provided as aunitary rectangular ring or box 8311 which may closely surround theedges of the chip. Each support element has a top surface 8313 (FIG. 16)arranged to lie substantially coplanar with the front or top surface8322 of the chip. Thus, chip 8320 and support elements 8307 may bedisposed on a planar carrier 8315, and the thickness of the supportelements may be substantially equal to the thickness of the chip.

In assembling the interposer to the chip, the interposer with thevarious terminals and leads thereon is positioned on the chip so thatthe slots, and hence the leads, are aligned with the peripheral contactson the chip. Each securement element 8361 overlies one support element8307, and is at least partially supported by such element. A bondingtool is then advanced into each slot 8367 and engaged with theperipheral contact leads 8350 and with the outside terminal leads 8372,so as to force each such lead into engagement with one of the peripheralcontacts 8330 on the chip. Heat, pressure and ultrasonic energy may beapplied through the tool to promote bonding. The arrangement of theleads within the slots greatly facilitates the bonding operation.Bonding tool 8355 may be advanced into one of the slots 8367 and movedalong the length of the slot so as to bond all of the leads to all ofthe peripheral contacts 8330 aligned with that slot. This process may berepeated for each slot 8367. The tool may engage and bond many leadssimultaneously.

After the leads have been bonded to the contacts, a low modulusdielectric encapsulant (not shown) is applied. In an alternativeassembly process, the compliant layers 8340 and 8303 may be formed bythe encapsulant. Thus, the encapsulant may be applied so as to penetratebetween the interposer (not shown) and the chip to form compliant layer8340 between the interposer and the chip. The encapsulant may alsopenetrate between securement elements 8361 and support elements 8307 toform compliant layers 8303 and penetrate into slots 8367 to cover leads8374 and 8350. The encapsulant may be introduced under pressure in aliquid or flowable state and then cured. The interposer, chip andassociated elements may be disposed in a mold during this process, andthe mold may clamp the waste areas 8383 of the sheet or tape (FIG. 17)so as to limit the flow of the encapsulant. The encapsulant may beinjected under pressure using standard injection molding technique.After encapsulation, the assembly illustrated in FIGS. 16 and 17 may beseparated from the tape and mounted to a substrate in substantially thesame way as the assemblies discussed above. Thus, both the outsideterminals 8372 and the central terminals 8348 may be bonded to contactpads on the substrate.

The assembly illustrated in FIGS. 16 and 17 provides good reinforcementof the leads during manufacture. Also, the outside terminals provideincreased connection capacity. Although the securement elements andoutside terminals extend outwardly beyond the peripheral contacts on thechip, this outward extension or "fan-out" is minimal. Preferably, theassembly with securement elements and outside terminals occupies an areain the plane parallel to the chip surface no more than about 1.5 times,and desirably no more than about 1.2 times, the area occupied by thechip itself.

As shown in FIG. 18, an interposer 8436 according to a furtherembodiment of the invention is provided with securement elements 8461,slots 8467 and outside terminals 8472 similar to the correspondingcomponents discussed above with reference to FIGS. 16 and 17. Outsideterminals 8472 are disposed on the second surface of each securementelement, i.e., on the surface directed away from the semiconductor chip8420. Interposer 8436 also has central terminals 8448 on the secondsurface of the interposer. Each central terminal 8448 is connected to apartial lead 8450 and bonding terminal 8452. Likewise, each outsideterminal 8472 is connected to a similar partial lead 8475 and bondingterminal 8477. There are rows of bonding terminals 8452 and 8477 on bothsides of each slot 8467. The bonding terminals are connected to theperipheral contacts 8430 on chip 8420 by a wire-bonding operationsimilar to that discussed above with reference to FIG. 13. Here again,disposition of the bonding terminals in rows facilitates thewire-bonding operation.

Chip 8420 also has central contacts 8431 disposed in the central regionof the chip front surface. Interposer 8436 has a hole 8480 encompassingthese central contacts. Some of the bonding terminals 8452 associatedwith certain central terminals 8448 are disposed adjacent the edges ofhole 8480. These bonding terminals are connected by wire bonds to thecentral contacts 8431 of the chip, so that the central contacts as wellas the peripheral contacts 8430 will be connected to the substratethrough the central terminals 8448 of the interposer.

Assemblies according to the invention may include additional elementsfor mechanical and electrical protection. Thus, a thin electricallyconductive grounding layer such as a metallic layer may be incorporatedin the interposer to electrically isolate the terminals from the chip,and to provide better control of impedances in leads extending along theinterposer. Such a conductive layer must be separated from the terminalsby a dielectric layer. The interposer itself may include multiple layersof terminals and leads separated from one another by intermediatedielectric layers. Such an arrangement allows the leads on theinterposer to cross over one another without contacting one another, andallows for more leads and/or wider leads in a given area. The topmostlayers of such a multilayer interposer may have holes aligned with theterminals of the lower layers, to provide access to these lower-layerterminals and permit connection to a substrate.

The components illustrated in FIG. 20 are similar to those depicted inFIGS. 16 and 17. Thus, the structure includes an interposer 8736 andsecurement elements 8761 defining slots 8767 therebetween, only one suchsecurement element and slot being visible in FIG. 20. The outsideterminal leads and peripheral leads include portions 8754 extendingacross the slots. Each such lead portion extends into the slot fromabove the compliant layer 8703 of the associated securement element andabove the compliant layer 8740 of the interposer. In the conditionillustrated in FIG. 16, before bonding of lead portions 8754 toterminals 8730 of the chip, these lead portions are substantiallyplanar. That is, they extend substantially in a plane parallel to theplane of interposer 8736 and hence parallel to the plane of chip frontsurface 8722 when the interposer overlies the chip. Each such lead iscurved in this horizontal plane, in the direction of elongation of theslot. Thus, each such lead includes end portions 8780 and 8782 at theedges of the slot, adjacent securement element 8761 and interposer 8736respectively. Each lead portion 8754 further includes a middle portion8784 adjacent the center of the slot and overlying one of the peripheralcontacts 8730 on chip 8720. Each such middle portion 8784 is offset fromthe imaginary axis connecting ends 8780 and 8782. As shown in FIG. 20,the offset is in the direction of elongation of slot 8767. During theassembly process, a tool 8786 is advanced into slot 8767 to bond leadportion 8754 to chip peripheral contact 8730. The tool engages themiddle portion 8784 of each lead portion, and forces the middle portiondownwardly into engagement with chip contact 8730. Because the middleportion is offset from the axis connecting ends 8780 and 8782, thisdownward motion of the middle portion can be accommodated by acontrolled twisting motion of the ends. The middle portion 8784 may alsobend downwardly to some degree. This structure provides a controlleddownward motion of middle portion 8784. As each lead portion 8754 isretained at ends 8780 and 8782 during this operation, the portions willremain in the desired positions and hence will be properly aligned withchip contact 8730. As all of the middle portions 8784 are offset in thesame direction, the offsets in the lead portions do not appreciablyincrease the required spacings between lead portions 8754 along thelength of slot 8767. Moreover, these offsets, lying in the plane of theinterposer, can be formed without any separate embossing or bendingoperation, in the same etching operation used to form the leads. Thebonding tool may engage and bond the middle portions of several leadssimultaneously.

As illustrated in FIGS. 21 and 22, a connection component 930 for use inproviding terminals on the rear or bottom surface of a chip includes agenerally cruciform, unitary sheet comprising a generally rectangularbacking element 932 and flaps 934 projecting from the edges of thebacking element. The sheet has a layered structure including aconductive layer 936, an insulating layer 938 and a further insulatinglayer 940 on the opposite side of conductive layer 936. Layer 938defines a first surface 942 of the connection component, whereas layer940 defines a second surface 944. A set of terminals 946 are disposed onthe first surface 942 of the connection component in a central region ofthe backing element 932. These terminals may be disposed in arectilinear, grid-like array. Although only a few terminals are shown inFIG. 21 for clarity of illustration, several hundred terminals may beprovided on a typical component.

Leads 948 are also formed on the first surface 942 of connectioncomponent 930, each such lead being formed integrally with one terminal946 and electrically connected thereto. Leads 948 extend outwardly, awayfrom backing element 932 on flaps 934, and project to the extremities ofthe flaps. Thus, each such lead 948 includes a flap portion extendingalong the associated flap, and a central portion extending from theinner margin of the flap to the associated terminal 946. The thicknessof the various layers constituting connection component 930 is greatlyexaggerated in FIG. 22 for clarity of illustration. In practice, each ofthese layers has the minimum thickness required to meet electricalrequirements. Desirably, insulating layers 938 and 940 have the minimumthickness required to provide freedom from pinholes and breaks in theinsulation, whereas conductive layer 936 and leads 948 have the minimumthickness required for electrical continuity and to provide a relativelylow resistance current path. Preferably, each of the insulating layersis less than about 0.5 mm thick, and more preferably, less than about0.25 mm thick, whereas conductive layer 936 preferably is less thanabout 0.1 mm thick and each of leads 948 preferably is less than about0.1 mm thick. Connection component 930 may be formed from substantiallythe same materials, and in substantially the same manner, as the tapeused for tape automated bonding processes. Thus, insulating layers 938and 940 may incorporate conventional polymeric dielectric materials suchas polyimide, whereas layer 936, conductors 948 and terminals 946 may beformed from copper or other metals. The pattern of terminals andconductors may be formed by photochemical etching or depositiontechniques similar to those used in the manufacture of tape automatedbonding tapes and flexible printed circuits.

Component 930 may be used with a box-like element 950 shown in FIGS. 23and 24. Box element 950 includes four support elements or walls 952arranged to form a generally rectangular ring and a floor element 954extending across the interior of this ring so that walls 952 and floorelement 954 cooperatively define a rectilinear closed-bottom box havingan interior space 956 open at the top (the side visible in FIG. 23). Thebox has length l and width w slightly larger than the correspondingdimensions of chip 920, whereas the depth d of the box desirably isslightly greater than the thickness of chip 920, i.e., slightly greaterthan the distance between surfaces 922 and 924 of the chip. Each supportmember or wall 952 has a projection 958 extending downwardly, beneathfloor element 954, so that projections 958 and floor element 954cooperatively define a further open interior space 960 on the bottomside of floor element 954. The floor element has several holes orapertures 962 extending through it, between spaces 956 and 960. Space960 is shallower than space 956. Box element 950 may be formed fromsubstantially rigid materials such as thermoplastics or thermosettingpolymers, glass, ceramics glass-ceramic materials, polymer-matrixcomposites and metal-matrix composites, and metals, metals and polymersbeing preferred.

In a fabrication process according to one aspect of the invention, aresilient, compliant layer 964 (FIG. 25) formed from a relatively lowelastic modulus material is provided in the lower or downwardly facingspace 960 of box element 950. Preferably, this low-modulus material haselastic properties (including modulus of elasticity) comparable to thoseof soft rubber, about 20 to about 70 Shore A durometer. Compliant layer964 has holes 966 interspersed with masses 968 of the low modulusmaterial. Layer 964 may be formed from a sheet of solid elastomer bypunching or perforating to form holes 966, and then inserted into thelower space 960 of box element 950 and fastened in place by adhesivematerial 970 extending through holes 962 in the floor element 954 of boxelement 950. A portion of this adhesive material may partially or fullycoat the top surface of floor element 954 so as to provide some degreeof surface adhesion or tack on the top surface of the floor element.Alternatively, compliant layer 964 may be formed by molding in placewithin the lower space of the box element. Thus, the elastomericmaterial may be introduced in a fluid condition and chemically or heatcured to a resilient state. Where the compliant layer 964 is formed inthis fashion, some portion of the elastomeric material may protrudethrough holes 962 in much the same way as adhesive material 970. Thisserves to fasten the compliant layer to the undersurface of the floorelement. The compliant layer may also be applied by silk-screening. Inyet another alternative procedure, the compliant layer can simply beplaced within the lower space of the box element without fastening it tothe box element.

In the next stage of the assembly process, connection component 930 isjuxtaposed with box element 950 so that the second surface 44 of theconnection component confronts the exposed or bottom surface ofcompliant layer 964, and so that the backing element 932, is alignedwith floor element 954 and compliant layer 964. At this stage of theprocess, each flap 934 of connection component 930 projects outwardlybeyond walls 952 and extends across the lower extremity of oneprojection 958. Thus, the central region of the backing element bearingterminals 946 is aligned with compliant layer 964, the terminals facingdownwardly, away from the compliant layer and floor element 954. Thearrangement of masses 968 in compliant layer 964 is selected to matchthe arrangement of terminals 946. As best illustrated in FIG. 26(showing a later stage of the process) each terminal 946 is aligned witha mass 968 of the low modulus material whereas the holes 966 in layer964 are aligned with spaces between terminals 946.

In the next stage of the manufacturing process, flaps 934 are bentupwardly alongside the walls or support elements 952 of box element 950.Thus, each flap 934 and the flap portions 48 of the conductors on suchflap extends upwardly alongside the associated wall 952. The extremityof each flap is bent inwardly over the uppermost margin of theassociated wall 952. Thus, as seen in FIG. 25, the extremity of flap934a is bent inwardly at the upper extremity of wall 952a. Likewise,flap 934b extends upwardly along side wall 952b as illustrated in FIG.26 and is bent inwardly over the upper most extremity of wall 952b.Thus, the extremities of conductors 948 adjacent the edges of the flapsare disposed along the top edges of walls 952, remote from floor element954 around the top opening of space 956. Conductors 948 extenddownwardly alongside the walls of the box element to terminals 946,which are disposed beneath the box element. As connecting element 930and hence flaps 934 are flexible, the bending operation can be performedreadily. The extremities of the flaps overlying the top edges of walls950 are bonded to the tops of the walls.

A layer of a preferably flexible dielectric material is applied as asolder mask layer 972 covering the downwardly facing first surface ofbacking element 932. Solder mask layer 972 is provided with apertures974 aligned with terminals 946 of the backing element. This solder masklayer may be formed by molding or by selective curing of an elastomericmaterial. For example, the material may be applied in a flowable,uncured state and then cured by radiant energy. The radiant energy maybe applied selectively so as to cure all portions of the layer exceptthose portions overlying the terminals 946. Subsequent to this selectivecuring, the uncured portions may be removed. Alternatively, the soldermask may be applied as a solid layer and punctured to expose terminals946. As discussed further hereinbelow, solder mask layer 972 and may beomitted in certain cases.

The assembly at this stage constitutes a receptacle adapted to receive asemiconductor chip. These receptacles can be prefabricated in massproduction and distributed to semiconductor chip manufacturers andusers. Alternatively, the receptacle can be fabricated immediatelybefore it is united with the semiconductor chip.

The receptacle is united with a semiconductor chip 920 by first placingchip 920 (FIG. 26) into the top or upper space 956 of box element 950,so that the front face 922 of the chip faces upwardly, away from floorelement 954 and backing element 932. The chip 920 may be temporarilyretained in position within the receptacle by the adhesive 970 on thetop surface of floor element 954. In this position, the edges 926 of thechip confront the support elements or walls 952 of the box element. Chip920 is of substantially the same type as illustrated in FIG. 11. This,chip 920 has contacts 928 disposed on its front surface 922, thecontacts being arranged in rows adjacent the edges 926 of the chip.Flaps 934, and hence lead portions 948 on the flaps, extend upwardlyalongside edges 926 of the chip, so that the leads on each such flapextend to the vicinity of one row of contacts 928 on the chip. Each rowof contacts 928 is positioned immediately adjacent to the extremities ofleads 948 on one of flaps 934. The front surface 922 of the chip, andhence contacts 928, are disposed at approximately the same height abovefloor element 954 as are the extremities of leads 948, although theextremities of the leads may be elevated slightly above surface 922.

While the chip is in this position, the contacts 928 are electricallyconnected to leads 948 by wire bonding the contacts to the adjacentextremities of the leads. In the wire bonding operation, fine wires 974are connected between contacts 928 and lead portions 948, therebyelectrically connecting each lead portion 948 to one contact 928 in theadjacent row of contacts. In effect, wires 974 merge with lead portions948 to form a composite lead extending from terminal 928, around onewall element 952 and downwardly alongside the edge 926 of the chip toone terminal 946 on the backing element 932. The process of wire bondingper se is well known in the electronics art and need not be described indetail herein. Briefly, this process utilizes a movable wire dispensingand bonding head. The head is brought into engagement with one of theelements to be connected and an end of a fine wire is bonded to suchelement. The head is then moved while paying out the wire until itreaches the other element to be connected, whereupon the wire is bondedto such other element and cut, leaving the wire in place. Wire bondingprocesses typically are controlled by detecting the relative positionand orientation of the components to be connected and then controllingthe wiring bonding head accordingly so as to bring the wires intocontact with the desired elements. This allows the desiredinterconnections to be made even where the relative positions of thecomponents to be connected differ from the nominal positions. Typically,the relative positions and orientations of the components are detectedby robot vision systems, such as television-based pattern recognitionsystems. These techniques desirably are used in the wire bonding step ofthe present method. Where such techniques are employed, it is notessential to provide great precision in the positioning of chip 920 orin the positioning of lead portions 948. This minimizes the need forclose control of the bending operation discussed above.

After the bonding wires 974 have been attached, a pad 975 of a soft,thermally conductive material, such as silicone with a thermallyconductive filler, is placed atop the front surface 922 of the chip. Thepad covers the central portion of the chip front surface, remote fromcontacts 918 and wires 974. A layer of an encapsulant 976 is appliedover the front face 922 of the chip. The encapsulant, which desirably isa soft, dielectric material covers the bonding wires 974, the contacts928 and the extremities of the lead portions 948 disposed atop the walls952. The encapsulant desirably also penetrates into and at leastpartially fills spaces between the edges 926 of the chip and theconfronting walls 952 of the box element. A cover 978 is then placedover the top of the assembly. Cover 978 may be a box-like metallicelement, commonly referred to as a "chip can", or else may be molded ina place on the assembly from a polymeric material such as an epoxy.Cover 978 may be united with the periphery of the solder-mass layer 972so as to seal the assembly against subsequent contamination. Encapsulant976 contacts the front surface 922 of the chip and also contacts cover978, thus providing a path for heat transmission from the chip to thecover. This facilitates heat transfer from the chip to the surroundings,outside the assembly, during operation of the chip. Cover 978 alsocontacts layer 975, further facilitating heat transfer.

The assembly desirably is tested before being used as part of a largerassembly. The assembly desirably is tested in substantially the same wayas discussed above using an electrical testing fixture having numerouspins or probes connected to an appropriate test circuit and rigidlymounted to a common fixture or support. To provide a reliable test, thenumerous pins or probes on the test fixture must be held in contact withthe respective terminals 946 at the same time. In this arrangement aswell terminals 946 can be independently displaced towards chip 922. Suchdisplacement permits continued movement of the test fixture and assemblytowards one another, until all of the pins are engaged with theirrespective terminals 946. Each terminal 946 will be biased against theassociated pin of the test fixture by the resilience of compliant layer.This assures reliable contact and a reliable test. As discussed above,the configuration of compliant layer 964 contributes to this action.Each mass 968 of low modulus material provides backing and support forthe terminal 946 aligned therewith. As the pins of the test fixtureengage the terminals, each mass 968 is compressed in the verticaldirection and therefore tends to bulge in horizontal directions,parallel to the plane of the chip. Holes 966 provides space for suchbulging. Compliant layer 964 need only provide for sufficient movementof terminals 946 to accommodate tolerances in the test equipment and inthe assembly itself. Typically, about 0.0005 inch (0.125 mm) or lesscompliance is sufficient. For example, compliant layer 964 may be about0.008 inch (0.2 mm) thick.

After testing, the assembly is mounted to a substrate 988 (FIG. 26)having electrical contact pads 990, using techniques similar to thoseused for mounting the assemblies discussed above. For example, theassembly may be placed on the substrate so that the apertures 974 insolder mass layer 972 and terminals 946 are aligned with the contactpads 990 of the substrate. Masses of an electrically conductive bondingmaterial 991 such as a solder or an electrically conductive adhesive maybe disposed between the terminals 946 and the contact pads 990 of thesubstrate. These masses may be caused to flow and to bond with theterminals and contact pads, in the same manner as discussed above.

Because terminals 946 are disposed at substantial center to centerdistances, standard surface mount techniques can be used withoutdifficulty. In this regard, it should be appreciated that terminals 946are distributed over an area approximately equal to the entire area ofthe chip bottom surface 924. By contrast, contacts 928 of the chipitself are concentrated in rows around the periphery. Thus, the centerto center distances between the terminals 946 may be substantiallygreater than the center to center distances between contacts 928. Intypical applications, electrical connections for a chip having asubstantial number of input and output terminals, commonly referred toas a "I/O count" can be achieved with 10-25 mil (250-625 micrometer)center to center distances.

The composite leads including lead portions 948 and bond wire 974provide reliable interconnections between contacts 928 and terminals946. Because the electrically conductive layer 936 of connecting element930 extends upwardly, alongside the chip with lead portions 948, leadportions 948 have predictable, controlled impedance. This reliableelectrical performance is also enhanced by the predictable geometricconfiguration of lead portions 948. Each lead portion 948 has apredetermined width and is located in a predetermined position relativeto the adjacent lead portions. These relative positions and widths arefixed when the connecting element 930 is made. Although the compositeleads do include bonding wires 974, these bonding wires are so shortthat they do not introduce appreciable unpredictable capacitance orinducence.

The assembly thus provides a compact, rugged and economical chipmounting. The entire assembly occupies little more area (in the plane ofthe chip) than the chip itself. As the leads and flaps extend alongsidethe chip, in close proximity to the edges of the chip, they do notsubstantially increase the area occupied by the assembly. Also, becausethe assembly can be pretested before mounting to the substrate, highquality can be assured. The methods and structure discussed above can bevaried in numerous ways. Also, solder mask layer 972 may be applied atany stage in the process. If desired, this layer could be formed as partof connection element 930 or applied after the remaining components ofthe assembly, as by molding in place so that solder mass layer 932contacts cover 978.

The configuration of box element 950 can be varied from thatillustrated. The floor element 954 can be omitted entirely, or else thefloor element may include only small tabs projecting inwardly from thewalls 952 so as to support the chip only at its edges or corners. Ineither case compliant layer 964 will be in direct engagement with thebottom surface of the chip and with the backing element. Alternatively,the holes 962 in the floor element 954 may be omitted. The downwardprojections 958 of the walls 952 may be omitted, so that the wallsterminate flush with the floor element or flush with the bottom surfaceof the chip if the floor element is omitted. The bottom edges of thewalls may be provided with chamfers or radii to prevent damage to theconnection component 930 when the flaps are bent upwardly. The boxelement may be provided with supports, such as legs at the corners ofthe box element, projecting downwardly for engagement with thesubstrate. In this case, the box element will serve to support the chipabove the substrate, thereby preventing crushing of the solder jointsduring manufacturing procedures or in use. This arrangement isparticularly useful where a heat sink is forcibly held in engagementwith the front surface of the chip. Also, the box element can beemployed as part of a hermetic sealing arrangement around the chip.

The compliant layer 964 disposed adjacent the backing element can extendoutwardly to the outer surfaces of the walls or support elements 952, sothat a portion of the compliant layer is interposed between the loweredge of each such wall or support element and the backing element. Thisarrangement is particularly useful when some of the terminals 946 aredisposed on that portion of the backing element aligned with the bottomedges of the walls.

Where the coefficient of thermal expansion of the box element differssubstantially from the coefficient of thermal expansion of the chip, thebonding wires 974 can flex to compensate for relative movement of thechip and the lead portions at the extremities of the flaps, overlyingthe top edges of the walls. In those cases where the flap portions ofleads 948 are bonded directly to the contacts on the chip as discussedbelow, those flap portions of the leads may be flexible to providesimilar compensation. Where the coefficient of thermal expansion of thebox element differs substantially from that of the substrate, thebacking element preferably is not bonded to the bottom of the boxelement except through the compliant layer. This permits the flaps toflex and the backing element to move relative to the box element andabsorb differential thermal expansion.

The configuration of the heat transfer elements can be variedconsiderably. Thus, the thermally conductive pad or layer 975 mayinclude a metallic slab bonded to the front or top surface of the chip.Such a metallic heat sink may include fins, plates or projections tofurther facilitate heat transfer. A plurality of chips can be engagedwith the same heat sink. Essentially any heat sink which can be usedwith conventional face-up chip assemblies can be employed.

The backing element and flaps may include more than one layer of leads,so as to accomodate particularly complex interconnection requirements.Also, more than one flap may be provided at each edge of the backingelement, and these multiple flaps may extend in superposed relationalong the edge of the chip or along the wall of the box element.

As illustrated in FIG. 27, the box element may be omitted. Thus, theflaps 9134 of connecting element 9130 may be folded upwardly, alongsidethe edges 9126 of chip 9120, without intervening wall members. Also,compliant layer 9164 may be disposed directly between backing element9132 and the bottom or rear surface 9124 of the chip 9120, without anyintervening floor element. In the arrangement shown in FIG. 27, eachflap 9134 not only extends upwardly along the side the edge 9126 of thechip but also extends inwardly, over a marginal portion of the chipfront surface 9122 adjacent the edge 9126. Each flap has a slot 9137overlying a row of contacts 9128 on the chip. The extremities 9149 oflead portions 9148 extend across this slot and hence overlie the chipcontacts 9128. In the assembly process, extremities 9149 can be bondeddirectly to terminals 9128 by techniques similar to those discussedabove with reference to FIGS. 16 and 20. To facilitate the bondingoperation, extremities 9149 may be curved in directions parallel to thelength of slot 9137 so as to permit them to deflect downwardly andengage contacts 9128 more readily under the influence of bonding tool9151. In the manufacturing process, the connecting element 9130 andcompliant layer 9164 are assembled to chip 9120, and the flaps 9134 ofthe connecting element are folded directly upwardly, alongside the edges9126 of the chip. The extremities of flaps are then folded inwardly overthe front surface of the chip. The assembly illustrated in FIG. 27 mayalso be provided with a solder mask layer, housing and encapsulant asdiscussed above.

The arrangement of FIG. 28 is similar to that discussed above withreference to FIG. 27, in that the extremities of flaps 9234 are foldedover the front surface 9222 of the chip, thereby positioning theextremities 9249 of lead portions 9248 over contacts 228 on the chip.Here, however, the connecting element includes vias 9251 extendingthrough it from beneath each lead extremity 9249 to the second surface9244 of the flap, i.e., the surface opposite from the first orlead-bearing surface 9242. Each such via is filled with an electricallyconductive bonding material such as a thermocompression bonding alloy9253. Bonding material 9253 is activated by heat or pressure, usingconventional bonding techniques, to bond each lead extremity 9249 to onecontact 9228 on the chip. The electrically conductive layer 9236 of theconnecting element terminates remote from vias 9251, so that theelectrically conductive layer does not make an electrical connectionwith the conductive material 9253. If desired, the conductive layer 9236may be extended to one or a few of vias 9251 so as to provide a groundconnection to layer 9236. That is, one of leads 9248 may be connected toa terminal (not shown) which in turn is connected to a ground on thesubstrate, and layer 9236 may be grounded through that lead.

As an alternative to thermocompression or other conventional bondingtechniques, the leads can be connected to the contacts on the chip byusing a so-called "Z-conducting" adhesive. Such materials ordinarilyinclude electrically conductive particles selected so that when thematerial is applied in a thin layer, it will have appreciable electricalconductivity in the direction through the layer but only insignificantconductivity in directions parallel to the layer. Z-conducting adhesivesmay also be used to connect the leads of the interposers discussed aboveto the contacts of the chips.

As illustrated in FIG. 29, a sub-assembly in accordance with the presentinvention may be mounted on another chip. For example, as illustrated inFIG. 29 contacts 9328 on the front surface 9322 of chip 9320 areconnected, through terminals 9346, to contacts 9391 of a semiconductorchip 9393. Thus, chip 9393 itself serves as the substrate for mountingthe assembly incorporating chip 9320. Chip 9393 in turn is connected viaconventional wire bond leads 9395 to a further substrate and hence toother electronic elements. Conversely, a further chip 9377 is mounted soas to overlie the front surface of chip 9320. An interposer 9379 isdisposed on chip front surface 9322. This interposer has terminals 9381connected to some of the contacts 9328 on the chip front surface viaflexible leads. The interposer itself is flexible and includes acompliant layer 9383 disposed between terminals 9381. Those terminals inturn are connected to terminals 9356 of a further sub-assembly, which inturn are connected to contacts 9338 of chip 9377. Thus, chips 9320 and9377 are interconnected in a stacked circuit assembly which in turn ismounted on chip 9393. Any number of chips can be interconnected in sucha stacked assembly.

In an assembly according to a further embodiment of the invention,illustrated in FIG. 30, the orientation of the sheet-like connectioncomponent is reversed. That is, the lead-bearing or first surface 9442faces toward chip 9420. Terminals 9446 are exposed through holes 9473extending through the insulating layers 9440 and 9438. The conductivelayer 9436 disposed between these insulating layers terminates remotefrom holes 9473, so that the two insulating layers merge with oneanother at the boundaries of the holes and insulate the holes from layer9436. Thus, bonding material can be introduced in holes 9437 so as toconnect terminals 9446 to a substrate. Also in this arrangement, theextremities 9435 of flaps 9434 are bent outwardly, away from the chip,and the walls or support elements 9452 are disposed outside of theflaps. That is, the flaps lie between support elements 9452 and thechip. Compliant layer 9464 immediately underlies terminals 9446.

In a further variant (not shown) the support elements or walls can beintegral with the connecting element, and particularly can be integralwith the flaps. Thus, the connecting element can have relatively stiffregions constituting the flaps and a flexible region constituting thecentral or backing element. The stiff regions constituting the flaps canbe bent upwardly so as to form a self-supporting structure. As in thearrangement discussed above, this provides a generally box-like orcup-like structure having an open top with lead portions disposed aroundthe periphery of the opening for receiving a chip and connectingthereto.

As will be readily appreciated, numerous further variations andcombinations of the features discussed above can be utilized withoutdeparting from the present invention as defined by the claims. In onesuch variant (not shown) the backing element is provided substantiallyas discussed above, but the flaps and the lead portions on the flaps areomitted. In this arrangement, the bonding wires constitute the principalportion of each lead. The bonding wires extend downwardly, alongside theedges of the chip, to the backing element and join the backing elementadjacent to the rear or bottom face of the chip. In this arrangement,the bonding wires constitute the leads extending alongside the edges ofthe chip. This arrangement is distinctly less preferred because it doesnot offer the same degree of control over lead impedance as the otherarrangements discussed above. Thus, the foregoing descriptions of thepreferred embodiments should be taken by way of illustration rather thanby way of limitation of the invention defined by the claims.

What is claimed is:
 1. A method of making a plurality of semiconductorchip assemblies comprising the steps of:(a) providing a wafer includinga plurality of semiconductor chips, each such chip having a plurality ofcontacts; (b) assembling a flexible sheetlike dielectric element bearinga plurality of terminals to each said chip whereby one said chip isassociated with each said dielectric element and connecting eachterminal of each said dielectric element to one contact on the chipassociated with that dielectric element by a flexible lead so that eachsaid flexible sheetlike dielectric element remains movable with respectto the chip associated with that dielectric element and each terminal ismovable with respect to the associated contact; and then, (c) severingthe individual chips from said wafer after said assembling step so thateach severed chip has the dielectric element associated with such chipand the terminals of that dielectric element attached thereto.
 2. Amethod as claimed in claim 1 wherein each said dielectric element hasleads prefabricated thereon prior to said assembling step, the step ofconnecting each terminal to one contact on the chip including the stepof connecting the prefabricated leads to the contacts.
 3. A method asclaimed in claim 1 wherein said assembling step is performed so thateach said terminal on each said dielectric element is moveable towardsthe surface of the chip associated with that dielectric element.
 4. Amethod as claimed in claim 1 further comprising the step of providing acompliant element between each said dielectric element and theassociated chip prior to said severing step so that each said terminalon each said dielectric element is moveable towards the chip associatedwith that dielectric element.
 5. A method as claimed in claim 1 whereineach said dielectric element has a plurality of apertures therein andsaid assembling step is performed so that said leads extend between saidcontacts and said terminals through said apertures.
 6. A method asclaimed in claim 1 wherein each said chip has a front surface with acentral region and a plurality of said contacts in a peripheral regionsurrounding the central region, said step of assembling said dielectricelements to said chips including the step of placing each saiddielectric element on the central region of the chip associated withthat dielectric element, said step of assembling terminals including thestep of connecting said leads so that said leads extend inwardly fromthe peripheral contacts of each chip to the terminals of the associateddielectric element.
 7. A method as claimed in claim 1 wherein saidterminals on each said dielectric element are disposed in an area arrayso that the terminals are substantially uniformly distributed within anarea of the dielectric element overlying a contact pattern area definedby the contacts of the chip associated with that dielectric element. 8.A method as claimed in claim 7 wherein said contacts on each said chipare disposed in an area array so that the contacts of each chip aresubstantially uniformly distributed within said contact pattern area ofsuch chip, said assembling step being performed so that each terminal ofeach dielectric element is disposed adjacent one contact of the chipassociated with that dielectric element and connected to such adjacentcontact.
 9. A method as claimed in claim 1 further comprising the stepof encapsulating said contacts and leads.
 10. A method as claimed inclaim 9 wherein said encapsulating step is performed prior to saidsevering step.
 11. A method of making a plurality of semiconductor chipassemblies comprising the steps of:(a) providing a wafer including aplurality of semiconductor chips, each such chip having a plurality ofcontacts; (b) assembling a flexible sheetlike dielectric element to eachsaid chip while the chip remains part of the wafer, whereby one saidchip is associated with each such dielectric element, each suchdielectric element having, prior to said assembly step, a plurality ofterminals and a plurality of prefabricated flexible leads connected tosaid terminals, said assembling step being conducted so that saidflexible sheetlike dielectric elements remain movable with respect tothe chips; (c) connecting each terminal of each said dielectric elementto one contact on the chip associated with that dielectric element byconnecting each said flexible lead so that each terminal is moveablewith respect to the associated contact; and then, (d) severing theindividual chips from said wafer after said assembling step so that eachsevered chip has the dielectric element associated with such chip andterminals of that dielectric element attached thereto.
 12. A method asclaimed in claim 11 wherein said step of connecting said flexible leadsincludes the step of connecting individual leads seriatim.
 13. A methodas claimed in claim 11 wherein said step of connecting said flexibleleads includes the step of connecting a plurality of said leadssimultaneously.
 14. A method as claimed in claim 11 wherein said step ofconnecting said flexible leads includes the step of deforming each saidlead.
 15. A method as claimed in claim 14 wherein said step of deformingeach said lead is performed so that after said deforming step, each saidlead on each said dielectric element is curved in directionsperpendicular to the face of the chip associated with that dielectricelement.
 16. A method as claimed in claim 11 wherein said step ofassembling said dielectric elements to said chips includes the step ofassembling a separate dielectric element to each said chip in saidwafer.
 17. A method as claimed in claim 11 wherein said step ofassembling said dielectric elements to said chips includes the step ofassembling a sheet incorporating a plurality of said dielectric elementsto said wafer so that each dielectric element in the sheet overlies onechip.
 18. A method of making a plurality of semiconductor chipassemblies comprising the steps of:(a) providing a wafer including aplurality of semiconductor chips, each such chip having a plurality ofcontacts; (b) assembling a flexible sheetlike dielectric element to eachsaid chip while the chip remains part of the wafer so that each suchdielectric element is associated with said chip; so that each saiddielectric element overlies a face of the chip associated therewith withsurfaces of each dielectric element facing toward and away from the faceof the associated chip; and that each such dielectric element remainsmovable with respect to its associated chip, each such dielectricelement having, prior to said assembly step, a plurality of terminalsand a plurality of prefabricated leads connected to said terminals; (c)connecting each terminal of each said dielectric element to one contacton the chip associated with that dielectric element by connecting theflexible lead so that each terminal is moveable with respect to theassociated contact and so that each said lead is curved in directionsperpendicular to the surfaces of the dielectric element andperpendicular to the face of the associated chip; and then, (d) severingthe individual chips from said wafer after said assembling step so-thateach severed chip has the associated dielectric element, leads andterminals attached hereto.
 19. A method as claimed in claim 18 whereinsaid connecting step includes the step of bending said prefabricatedleads of each said dielectric element in a direction perpendicular tothe face of the chip associated with that dielectric element.
 20. Amethod as claimed in claim 18 wherein said step of connecting saidflexible leads includes the step of connecting individual leadsseriatim.
 21. A method as claimed in claim 18 wherein said step ofconnecting said flexible leads includes the step of connecting aplurality of said leads simultaneously.
 22. A method as claimed in claim18 wherein said step of assembling said dielectric elements to saidchips includes the step of assembling a separate dielectric element toeach said chip in said wafer.
 23. A method as claimed in claim 18wherein said step of assembling said dielectric elements to said chipsincludes the step of assembling a sheet incorporating a plurality ofsaid dielectric elements to said wafer so that each dielectric elementin the sheet overlies the chip associated with that dielectric element.